I'm working with my current dm to try and incorporate the possibilities of an inter-planar arc to our current campaign but we are sort of hitting a wall on sorce info as the the appendix gives very little on the other planes and the only sourcebooks are from older editions (namely the planar handbook from 3.5). Are they're any sourcebooks with the 5E system out there?

Nope.  WotC has really neglected sourcebooks in general and has not released anything focusing on the planes.  Based on an Unearthed Arcana that was released last year, a Spelljammer campaign setting book might be in the works but nothing's been confirmed.  DriveThruRPG has a bunch of Planescape books from 2E available as PDFs, I'd recommend searching there.

Nothing official in 5e.  As 6th mentioned Planescape is available in both DriveThru and DMsGuild.  However, the pdf and reprints (I got some of both) are "decent".  You see, most of Planescape came as boxed sets, from what I can tell, and DriveThru and DMsGuild deliver them to you as giant singular books, which leads to pagination problem to see the least.  Planescape also has it's own lingo/argot that is fun if you like reading made up dialect, but if it's a bit of a learning curve if you're just looking for background.  Planescape also has this "factions" drama that's not at all mentioned in 5e lore to date, and frankly is sorta extraneous (there's a lot of drama in the planes already without having to create a total other drama amongst humanoid planescaping factions).

If I may recommend DMsGuild product by David Coulson the Codex of Infinite Planes.  Available in PDF and a variety of formats including a hefty hardcover.  He does a pretty great distillation of all the canonical "great wheel" planes, brings in some other lore like the sort of interstitial planes between elemental planes (so planes of ash, ooze, magma, ice, etc).  And recasts the fey wild and shadow fell into Echo planes of shadow, faerie, and dreams.  All the major planes get several pages with some inspiration for key locations and personalities and some solid adventure hooks.  To my reading and floating around wikis, the author seems to do a good job representing prior planar lore in other editions, particularly planescape, but doesn't bog the DM down and the whole book serves more inspirationally than a fiat as to how you should run planar adventures.  One caveat: no maps.  Author has a few other DMs Guild works under this infinite planes themes.  I've only got a Bestiary he put together as well as Heroes of the Infinite Planes which is basically a book of player options (backgrounds, subclasses, etc. no new races).  I get a lot of use out of Heroes of the Infinite Planes, I particularly dig the Gatecrasher Rogue and the Ooze Sorcerer.

Planescape books on the other hand have a hell of a visual style and some pretty unconventional but also incredibly inspirational maps.

Another option are two "sort of" Planescape books, both adventure anthologies from 2e: Tales from the Infinite Staircase, and The Vortex of Madness and Other Planar Perils.  You can mine a lot of planes working through those two anthologies (which can also be played as campaigns, Tales from the Infinite Staircase even has a sort of sequel but was too tied to Forgotten Realms pantheon for me to really dig or find useful).  I actually like the Infinite Staircase a lot more than the traditional Planescape Sigil and Outlands set up, though the two (three?) mechanisms actually coexist and the Outlands actually show up in Tales from the Infinite Staircase).  

Other caveat about 2e stuff, the stats are going to confuse you, plus Planescape had this whole magic changes depending on what plane you're on thing, which is fun, but not a thing I'd mess with in a new game.  It's not cool to have a player study spell lists and then be told their present plane changes the workings of the spell before they've ever cast it.

Baldur's Gate: Descent into Avernus was a campaign that took the PCs to the outer layer of the nine hells. The 3.5e Planar Handbook and the 3e Manual of the Planes were very good resources for interplanar adventures. You may be able to find pdfs on the DMs Guild.

Also, tomorrow WotC will be announcing their roadmap which will likely include Spelljammer as a campaign setting based on their recent UA releases.

Thank you for all the awesome info

I will also reiterate about the Planescape products on DMs Guild. The "Planes of..." series (Law, Chaos, Conflict, as well as Guides to Inner, Astral, and Ethereal) are the most in depth sources on the Planes I think TSR/WotC ever published. One nice feature of those, too, is that they are like 98% lore with very little mechanics. So, unlike the 3.5 Planar Handbook which is a lot of mechanical content (feats, prestige classes, etc.), the "Planes of..." series has almost none of that and is just pure lore. They hold up very well for later editions (and the use of planar cant terms is relatively light and easy to ignore).

Even products like the adventures from 2e are surprisingly sort of light on mechanics compared to later editions. They left much more open for DMs to fill in, which winds up making them a bit more useful for 5e than many 3e & 4e adventures, in my opinion. Also, from what I recall, a fair number of the Planescape adventures are less-geared towards the dungeon-crawl/many-small-encounters-per-day model versus the fewer-but-larger set piece kind of encounters that a lot of 5e games seem to be shifting towards, in my experience.

(Oh, and seconding the Tales of the Infinite Staircase adventure anthology. It's one of my all time favorites. I really dig the flexible format of the first and last adventure happen first and last, but all of the other adventures can be played in any order. However, NPCs are acting and events are happening throughout it so that whether you play a section early or later, it may be entirely different based on background events!)

Do we know if there will ever be books (source or campaign setting) based on the elemental planes or the celestial and neutral planes? I'd really love more detail about the psuedo-elemental sections as well as maybe mount celestia, elysium, or limbo. Maybe even the realm of dreams? What a cool campaign that could be. The witchlight,, Avernus, and ravenloft are really useful, and i'd love more options (I know spelljammer and the ethereal citadel one are coming, and that's great).

Official WotC published, no, but the upcoming Spelljammer book is all about extraplanar travel.

For some settings there is good extraplanar material though that I would trust. Keith Baker, the creator of the Eberron setting, and contributor to the WotC book Eberron: Rising from the Last War, wrote a supplement called Exploring Eberron that is a bunch of stuff they couldn't fit in the official book. One chapter has sections for each of the planes in Eberron.

Since WotC seem allergic to anything resembling an official or implied setting, I doubt we'll see anything as detailed as in earlier editions.

So do what you are doing - get inspiration from earlier books and create your own interpretations of the planes.

Is it likely Journeys trough the Radiant Citadel will include stories from other planes?