I wanted to know more about them too. You can make your own (legally) as long as your state allows possession, but you still have to get the stamp for it.
Here is a site that should answer just about all your questions, from forms to suppliers and even designs and performance.
SilencerTalk • Index page
Anything that is used to attenuate a firearm is considered a suppressor, and is regulated by the batfe under federal law. An adapter that is made to allow a oil filter to be used as a suppressor is considered a suppressor under federal law, and big fines and the big house are the penalty. There was a firefighter that used a 2-liter bottle taped to a 22 to silence the gun so he could shoot his dogs, all so he could go on vacation. As far as I heard, no federal charges were brought on him.
edit; the suppressor only works on the expanding gasses. It will do nothing about the sonic crack of a supersonic bullet, unless you slow that bullet down to subsonic velocities.