It looks like there are two Feats that would let my rogue have Find Familiar: Magic Initiate and Ritual Caster.  Does anyone have advice on which is better?  Magic Initiate would give me Find Familiar as the 1st level spell plus 2 more cantrips, but Ritual Caster allows me to learn more rituals later if I'm reading it correctly?

As a little background, due to the way the DM set the campaign up, we all have some wild magic(ish) floating around us, so I'm high Charisma as a Swash rogue with a level in Sorcerer - not sure if that makes any difference :)

If you are going to have a level of sorcerer then ritual caster may be the better option. Sorcerers don’t have ritual casting so you will with the feat, which will give more options as long as your DM makes ritual spells available to add. Magic initiate is good, and gives you cantrips but it only allows you to cast the spell 1st level spell once per day, and you cannot use your sorcerer spell slots to cast it again if something happens to your familiar. Ritual caster you can. 

I'm gonna put in another vote for Ritual Caster, especially since you're already taking a level of Sorcerer, which more than provides any of the benefits you would have gotten from Magic Initiate aside from the specific spell you're hoping to get.

I'll divert from popular opinion and say turn the Draconic Familiar gift option into a feat. The pseudodragon has strong senses and you don't need components should the worst happen. This version can also technically attack, if that means anything to you.

I would prefer ritual caster for this.

Also Sorcerers have a neat little way to get the most out of ritual  caster.  You can exchange a sorcerer spell for a  different one every level.  If you exchange it for a ritual spell you then add that ritual spell you your ritual book and then next level exchange it for another ritual spell.  The spell stays in your book so you still have access to it as a ritual.  This works well even if you are not finding a lot of spellbooks or scrolls.

It is worth noting that technically, a spellcaster cannot add a spell they intrinsically know into their ritual book.  You have to have the spell "in written form", which means  transcribing the spell into a scroll before you can then consume the scroll to add the spell to your ritual book.  There's some sense there, actually, especially for sorcerers - you may be able to produce the spell with your sorcerous might, but that doesn't mean you know how to do so as a lengthy, precise ritual.  You have to work out how to produce the same effect with a very different method of spellcasting, which can often take time and work.

Most DMs would likely handwave that, but it bears keeping in mind.

Get Ritual Caster to "acquire a ritual book".
Select Find Familiar and some other useful spell such as Identify for starters.
Go to a store and ask if they have stock or 2nd hand grimoires.  As previously discussed, there may relatively be an-abundance-of-grimoires-in-5e-worlds in comparison to that of scrolls.
If they don't have grimoires with all the spells you're looking for, grab a blank spellbook for 50 gp and 10 gp of ink for each spell you might hope a wizard might, using their own notation, might write your sought after spells within it.
Seek a wizard who can either let you look in their spellbook directly or who can be paid to write a copy of their spells into your spare spellbook as per wizard spellcasting :

YOUR SPELLBOOK
...
Replacing the Book. You can copy a spell from your own spellbook into another book—for example, if you want to make a backup copy of your spellbook. This is just like copying a new spell into your spellbook, but faster and easier, since you understand your own notation and already know how to cast the spell. You need spend only 1 hour and 10 gp for each level of the copied spell.
...

To help persuade the wizard you could even say that you'd give them back an intermediary book once you've copied the contents into your ritual book.
Try to get access to as many of the nine 1st level ritual wizard spells that you like/can and copy them into your ritual book at the cost of 2 hours and 50 gp worth of ink for each 1st level spell.

Quote from Yurei1453 >>

It is worth noting that technically, a spellcaster cannot add a spell they intrinsically know into their ritual book.  You have to have the spell "in written form", which means  transcribing the spell into a scroll before you can then consume the scroll to add the spell to your ritual book.  There's some sense there, actually, especially for sorcerers - you may be able to produce the spell with your sorcerous might, but that doesn't mean you know how to do so as a lengthy, precise ritual.  You have to work out how to produce the same effect with a very different method of spellcasting, which can often take time and work.

Most DMs would likely handwave that, but it bears keeping in mind.

 Yeah, and scribing a scroll generally requires Arcana proficiency, so you'd want to make sure you have that, as well.

My DM was willing to compromise on an intermediate time & cost for my Arcana-proficient Bard to write down a known ritual spell in not-scroll form - I thought of it as the "final rough-draft" state just before creating the self-casting scroll.

Alternatively, they could waive the Arcana check to copy a spell scroll that you wrote yourself, and/or allow you to copy it at a cost of 10gp & 1 hour per level. 

I’m going to agree with everyone and say ritual. Actually had this same question for my sorcerer. I planned to go find familiar and detect magic for the spells.