Who makes gun silencers, since they are illegal for all practical purposes, and how can they make a profit?

I know that they can be legal if you have the right licenses, but they cost too much unless your in it full time as a job, and then you can probably make one anyway. I just got curious, because I don't see how a buisiness could survive in that market, given how few people they have to sell to, and probably even fewer legit(legal) buyers.