Silenced rifles are not like what you see in the movies. There is still quite a report produced from the sonic boom/crack of the bullet. While the shooter's position is concealed, it is still obvious that a gun is being fired. They do make subsonic ammunition for rifles, but you also loose a lot of terminal ballistic effects. Subsonic ammo may not be as reliable or accurate either.
I've downloaded several videos off the Internet of a silenced AR15 (well it was actually a fully automatic M4) being fired. From what I can tell and from what first hand witnesses report, it is about as loud as a 22 LR.
I think most silencers are made out of a tube with washer shaped baffles inserted. The idea is to slow the escape of the gasses that leave the muzzle. I'm sure more complex designs exist, but I've not heard how they are set up. There are also integrally suppressed barrels meaning that the barrel and suppressor are manufactured as a single unit. These are probably the best suppressors.
Silencers are available for purchase to civilians as I understand it, but they require going through the class III (or was that II?) paperwork and tax stamp (was it $200?) rigmarole.
The cheapest, easiest, and most legal way to get a suppressor is to just turn off your hearing aid. [:P]