silencer .com is the suppressor site. They are very helpful, You may git ribbed about asking common questions, so put on your survivors thick skin. They are GREAT guys and extremely knowledgeable.
As others have said- The suppressor is the tax item not the gun. You can switch the can to anything you want and be ok with the tax man.
You can use the same can for multiple calibers. You WILL lose good sound damping when using a large caliber can with a small caliber rd. Among most suppressor users it is not recommended. Its kinda like a new sks owner. In the beginning they will bubba a sks even if the guys in the know tell them not to. Later they will return their sks to original configuration with a lesson well learned.
As others have said - Oil filter adapters do require a tax stamp. I see them as a novelty item.
.22 cans are not so expensive, That is the way to start. You can use then on rifles and pistols without much change over problems. Such as expensive threaded barrels and without higher pressure problems.
45. pistol cans are in high demand right now. They are VERY expensive. I have an older 45 can made for a Mac. It does the job well. The 45 does not have the crack down range anyway so it is very quiet for such a large caliber. They are not too expensive used. But putting one on a pistol would be like adding a brick on the end of the barrel. For me it would be nice to have a 45 pistol can but the complete rig just gets too expensive.
9mm cans are reasonable. There is a big price range on these cans with some being very quiet and some not so quiet. A mid range 9mm can could be used with a carbine or a pistol. Its not recommended but if multi use is what ya want, then this is the way to go. You could also use a 9 mm can on 38 and .22. A 9mm can on a Nagant would be very cool! Gas sealed so no escaping gases and more quiet.
Rifle caliber suppressors are very COOL. You can use a 223 suppressor on a .22 but not the other way around. Economy .22 suppressors can not handle the pressures of 223.
One thing to keep in mind is that rifle caliber suppressors do not silence the shot. They will make a shooters location unknown to something downrange. They do quiet the shot and with the right ammo they can be even more quiet down range.
A friend has a 308 suppressor and that thing is amazing! He has it equipped to use on an M1a, and several bolt actions. It seems to be most quiet on the M1A. You know a rifle has been shot, but you cannot tell if it is a .22 close by or a higher caliber father away. And you cannot tell where the shot came from.
Here is a little info, hope it helps.