You need to find a dealer who deals in them.
Do your research and then call the manufacturer who makes what you want. They should be able to steer you to a dealer in your state who can handle things for you.
Don't worry. The dealer will do all the paperwork. You just need to bring money.
So here are my thoughts on AR silencers. Fun, but expensive. Here's why.
An AR or any rifle, makes a lot of gas. A lot of very high pressure gas.
To put things in perspective. When I reload 9mm, I use about 5 gr of powder. When I reload .223, I use about 29 grains of powder.
The operating pressure of 9mm ammo is about 30,000 psi. The operating pressure of .223 is about twice that.
Why am I telling you this? Because it means that a suppressor for a .223 has to be larger and stronger. Which means it will be heavier. Much heavier.
A standard steel suppressor RUINS the balance of an AR if its put on anything other than a pretty short SBR. I have a 10.5" SBR with an older Gemtech M402.
Its 6 inches long, 1.37 inches wide and weighs 16 ounces.
The answer is to go into some of the more exotic materials. But that means more money. So. . . I guess here's my point. If you want a suppressor that you will actually enjoy shooting, prepare to spend for some exotic metals. Otherwise, you will end up with something you only take out for the novelty and not as a usefull addition to your gun.
To put the weight into perspective. I've got a standard Aluminum and steel 9mm can. It weighs10 ounces. I've also got a 9mm deGroat Nano thats all titanium. It weighs 4 ounces, about the same as my YHM mite all aluminum .22 can.
Remember, a silencer is a pressure vessel. Tensile strength at high temps is everything. This just happens to be something that Titanium is very good at.