You can get chains online, but I'd recommend you get a hold of your local driveline shop and see if they have one that's been sitting on a shelf for years. Pretty likely they'll sell you it at cost or something because they aren't likely to ever get rid of it with how obsolete that tcase is these days.
The slipping out of gear is most likely just the shifter linkage...it sucks and there isn't a real great solution out there. The detents are actually the shifter plates, and that thing is overly complicated and finnicky. You can threads on here where people made their own twin stick shifters and ORD sells cable shifters too, or you can make your own.
There aren't too many wear parts on there...
I use ultra black RTV for the mating surfaces,
there's a giant bearing on the front output which is the same as the 205 I believe and not likely worn at all,
there's a bunch of needle bearings on the output shaft that I've never had good luck sourcing (I have some in a bag I could mail you if you really got stuck),
needle bearing at the back of the output shaft, think it's the same as the 205
there's a big rear output bearing under the aluminum tailcone...it only wears out from poor lubrication like when some bonehead puts 90wt in there or if you have a part time kit and don't put it in 4x4 enough
I think the rear output had a thrust washer or shim in it for endplay, I just used a th350 input thrust washer and clipped the tang off it I think and it was close enough...It's been working fine and it only needs to last another year or so before that part of the case goes in the trashcan. I think it was blocking a bit of the oil splash but the rear output seal still gets enough back there to leak so I know the bottom of the bearing is submerged.
You're probably only really going to need to replace some needle bearings, the chain (they're like $100 though), probably that rear output bearing (since you have a part time kit), and maybe the front/rear output seals. Seals are the same as a 205 so they're probably easy to get with a part number, and I imagine between a place like US Bearing and/or a local driveline shop, you can find everything you need pretty easily.
I went through mine a couple years ago just to limp it along until I can afford a doubler, and found a local guy on Craigslist that built a doubler and traded him a six pack for the rear half of his 203, that's where I got my chain and some extra bearings, yoke, etc. Rebuilding the case is really very simple and not that involved, but getting it in and out of the truck is a pain in the ass for sure.
If all you want to do is replace the chain, you can remove the rear half of the case with it still in the truck depending on your floor clearnace. My '76 K5 has a removable tranny hump and this was very easy with just a 3/8" ratchet extension, and it's how I reassembled the case in the truck to reduce weight before I had a tranny jack. I tried to do this same thing on a Suburban once and the shape of the floor made it a totally different story though.
