You might also check your state game laws to see if they allow it.
My BIL use to be a class III dealer selling automatic weapons and suppressors (silencers) to law enforcement. Private individuals are $600 per transaction, if I remember correctly.
Like bigcountryextreme said, for quietness, stay subsonic, otherwise you have an expensive flash suppressor. I've shot SIONICS suppressors on M-16s and they aren't very effective except for muzzle flash.