No vendor will tell you what you need as they dont sell silencer parts. Its just the way it is.
Go join form1.org and start reading all the pistol threads. Then decide what you want, for length and diameter. The current hotness is 1.375 od and 1.255 id tubes, but if the extra .06 height over bore does not bother you then 1.5 tubes are just fine and there are more parts available.
You wont go wrong with a 6-7 inch tube stuffed full of dino Radisaurus cups with the right clips and the booster of choice. Then you just need to find the adapter for the booster and end cap of choice with a spacer to work. If you dont want to wait for the radisaurus cups even the 50's will do great.
edit, sorry I missed the ar part, not booster needed. I read the reply below and saw my error.