You have a couple different options.
You can either go with an LR308 or AR-10 pattern buffer system. I'll outline the differences below:
-AR-15 collapsible stock receiver extension.
-Short LR308 buffer.
-LR308 carbine action spring.
-AR-10 or VLTOR A5 collapsible stock receiver extension.
-AR-15 collapsible stock buffer.
-AR-10 action spring.
Unless you really need the shortest position/LOP on an AR-15 receiver extension, I'd go with the AR-10 pattern parts. The OBR rifles that come with collapsible stocks use AR-10 pattern buffer systems. For the CTR stock you'll want a 6-position receiver extension (standard AR-10 one from Armalite or VLTOR RE-10/A5SR), but if you go to a longer stock (such as ACS, STR, EMOD, ect.) you can use the more common 7-position receiver extension. For a buffer, I recommend starting with an H3 weight (or heaver if you'll be running suppressed).