Back in the early 80's, when I first started attending Gun Shows, suppressor parts kits were pretty common. If you had a registered tube, you could buy the kits and rebuild them. I don't know if any were ever sold to anyone who didn't have a registered tube.
But it has been a LOOOOONG time since I've seen any. Actually, though they are fairly straight-forward to build. I have been recently into paintball, and there have been a couple of cases where woods players actually got into trouble for building them for markers. Seems if they will fit on a firearm and suppress it, even a little, they are still regulated (even though made for a paintball marker).