I'm currently a level 4 Sorcerer. My DM has introduced me to another Sorcerer  (NPC) who has a familiar; some kind of dragon snake thingy. My DM asked me tonight if I was going to take the Find Familiar spell eventually so I could have one. I figured Sorcerer has it in their spell list but they don't. I'm sure he'll let me take it next level in some way, by an item or scroll or just homebrew that my Sorcerer has it in their list. Anyway, what all familiars can I pick from? My DM allows stuff from outside sources like Unearthed Arcana and Xanders, etc... so what are some other familiars that are in those that I could pitch to him as possible familiars aside from those listed in the PHB?

I am not aware of any official content that lets you choose additional forms of familiars. The only thing that lets you choose additional forms for your familiar, that I know of, are Warlocks with the Pact of the Chain feature which lets you choose pseudodragon,imp, quasit or sprite forms.

It seems your DM is allowing a homebrew version of the spell granting additional forms. You will need to speak with them about what options are then available to you.

As for gaining the spell your DM might be allowing it but there are non-homebrew ways to gain access to the spell regardless of class such as with the Magic Initiate or Ritualist feats or through use of spell scrolls.

Thanks! Yes, we discussed the scroll, however he said I had to be careful with it because his rules is that once it's done and I have the familiar and it dies, I can't gain one again. 

My Eldritch Knight has a Flying Snake - it’s 1/8 CR, same as the Poisonous Snake the spell lists by default. I’d say CR is reasonable enough limitation, since it can’t attack and is still puny enough to die in one hit.

I think Volo’s offers a variant that opens up the familiar pool, but I don’t have the book so I can’t check for myself.

Find Familiar is one of those areas that I really have trouble with varying editions.  I would imagine we will see additional forms at some point (though, how long is too long from the first printing of the PHB of the current edition?).  In previous editions, you had options like "Improved Familiar" that gave you the additional forms that Warlocks can choose.  Of course, to get those, the Warlock has to choose a very specific path... 

Personally, with a Sorcerer... I would be very leery of taking a Familiar.  You don't have many spell slots to begin with, so it potentially ties up a very valuable slot.  Familiars in 5e are ultra-squishy.  It's not like the old days where the Familiar gained health in tandem with the spellcaster (and I don't know why this was removed).  The action economy is horrible... it sounds cool, but you will never use it in combat, even though you can.  It costs you 10 gp every time you cast the spell, which at just a couple of hit points for the familiar is easily quite often (depending on your DM).  

Now, if you have need of a utility ability to scout things out and no one else can do it, this is a valuable resource to use.  Pull it out... send it off and then put it back in it's special dimension when you are done... or pull it out for roleplay purposes.  When the team is engaged, you don't want the familiar out... imho.  This is a great option for a Wizard who can just keep adding spells if they can find them.  Spells don't work that way for you... so make sure you absolutely have to have it before you replace a spell you will use multiple times per day every day.

I agree with the advice given by Grayfax but will add that if you really want Find Familiar consider taking it using a feat instead of a known spell. As Grayfax has highlighted as a Sorcerer your list of known spells is very limited and none of your metamagic will benefit it. The 10 gp cost isn't much, to be fair, but the use of the spell slot to keep bringing it back combined with limited selection of spells you can know can cause problems. Familiars are, indeed, incredibly squishy and their only benefit in combat, with one exception, requires sticking next to the enemy who can easily make it go bye-bye. You don't recover spell slots on short rests (unlike wizards or warlocks) so frequently using that spell slot to bring the familiar back is going to hamper you.

But do not despair - there are options! As I said consider taking feats. You can use Magic Initiate Wizard to get this spell and 2 cantrips. The cantrips can be cast like any cantrip and if you choose Find Familiar for the 1st level spell you can cast it once per day without needing spell slots.

Another option is choosing an owl form because it has Flyby - the only real use for a non-warlock familiar in combat is using Aid action to grant advantage on the first attack against the target which normally requires the familiar to stay in melee with the target because if it moves away they can attack your familiar with an Attack of Opportunity - familiars are easily hit, and even easier to go poof. However, the owl's flyby lets it move away without provoking attacks of opportunity and you do not need to remain in close quarters when using Aid so the owl can go in, distract with Aid, get out safely and you or your ally gets advantage on an attack against the target. The owl has an impressive fly speed too and being small makes it easy to get cover.

The best option if you do not want owl is taking the Ritual feat for the wizard class. This grants you the option to have book of rituals with 2 to start with from the chosen class. Wizards have more rituals than any other class and you can find or purchase these on your adventures to add to your ritual book. Any ritual in your ritual book can be cast as a ritual. Now you can ritual-cast the Find Familiar spell as often as you like - as long as you have the 10 gp components. At level 4 getting 10 gp is pretty easy. So, even if your squishy familiar gets 'killed' a lot you won't have to worry because on the next short/long rest you can bring them back. The only real worry then is if the familiar can get PTSD from dying a lot - personally I'd say no since it's an artificial mind and soul but, still, can't be nice.

The only real downside for these options is of course that you're taking feats for this instead of ability score increases or feats that could be more suited for combat (like spell sniper, warcaster, mobile, elemental adept, etc). So it really depends on how much you want the familiar.

Yeah, since you won't be able to cast it again unless the DM gives you another scroll or something, you'll have to be careful with the li'l fella. My Flying Snake has Flyby as well, though I don't think I ever tried to swoop it out of range and let the Help action ride, or maybe that got overruled. Usually having it glittering five feet overhead (I based it on a sunbeam snake, since my character wanted an inconspicuous familiar and comedically didn't get one) and having higher-priority targets around was good enough. Delivering touch spells just never seemed to work out initiative-wise.

We mostly used it to give me the occasional touch of pseudo-darkvision (bought goggles in the end), or to ride along with our scouting ranger. It could pick up things with blindsight that she couldn't, and sort of squeezed her to tip her off. Having the telepathic bond go both ways was a bit of a house rule, but it was just enough for the snake's stronger emotions such as "suddenly ambushed" or "got one-shot". And I could always do the share senses action in case of emergency. The more ambitious party members were trying to come up with a code to teach the snake so we could communicate back to the ranger (tap left shoulder for "coming to you", tap right for "retreat"...), but our DM nipped that ludicrousness in the bud and gave us earring walkie-talkies. Thanks, familiar!

This is kind of off topic, but...also directly related. I'm creating a  new character using the DND beyond website...and I'm a Warlock, Pact of the Chain with a Pseododragon...

My question is...how do I add a companion to my character? I feel like it's right in front of me and I'm just missing it, but...I'm not finding it!

Thanks in advance :)

Currently you would just write it in under the Notes section.  There is no framework for companion animals, etc. at this time.  It is being worked on, we just are not sure when that might happen.

Oh! Okay, I'm not just blind then. Cool ;) Thanks! :)

This seems a silly question, I have find familiar for my character as more of a plot point than really utilizing it as the spell intends, but am trying to determine if we can speak to each other, or if I am only able to speak and give commands to it?  My DM and I are interpreting the spell differently, I feel like we should be able to talk with each other, but he says it is only one way communication.