Technically, Pro/Carbine uses the Pro-Lite, 68 Carbine, Pro/Carbine threads, now it is dubbed "A5/X7" threads. Be extremely careful when choosing A5/X7 barrels though. I have found out if the barrel company do not cut the back end (threads area) of the barrel short enough, the front bolt will actually slam into the back of the barrel and get stuck thus causing cycling problems. So far I have found out the Deadlywind Null barrel does this, but Lapco and J&J are still good. I currently run a 12 inch J&J Ceramic on my Pro/Carbine.
And as a fellow Pro/Carbine owner, I can say there isn't much upgrades anymore. Depending on your style; Milsim or Tourney, there are somethings you could play around with. If you look at my sig, I have gone through the Milsim phase with my Pro/Carbine, and I am now at the speedball phase. Right now, for me, it's a matter of keeping it simple and basic. My current speedball setup on my "Pro/Carbine" has a custom cut feedneck/hand guard, Palmers Stabilizer, and an Allen Paintball grip frame. I also have a mini SP drop forward with a mini CP on/off ASA. More Pictures here.
As far as where to get parts, eBay would be your best bet. I still saw an Allen Paintball grip frame on there before, and I just recently picked up my J&J barrel from eBay too. Palmers still carries the Stabilizer, I highly recommend that, since the Vertical Air Adapter kit is pretty rare these days. The Palmers Stabilizer has been with my P/C since the CO2 days and now I am using HPA with it, and it still works fantastically.
Matt's Pro/Carbine page: Matt's Pro/Carbine Page
has some really cool mods you can do. It is where I learned the feedneck mod.
Hope this helps!