A suppressor is an NFA controlled item. Assuming it's even legal in your state, you'd have to go through a class III dealer and submit paperwork to ATF and wait for it to be approved before you can get it. You'd also have to pay for the $200 tax stamp and pay the dealer his transfer fee. The form itself would have had to have been signed by the chief law enforcement officer in your area unless you want to form a corporation or a trust.
Whatever model of suppressor you purchase would then have to be somehow fitted to your SKS, and you'd have to use subsonic ammo on it which I don't believe is commercially available, and if it is, it's probably pretty expensive. More than likely, it will also affect your point of impact, so you'd have to re-sight the rifle. I know people that spend a lot of time trying out all types of different combinations of suppressors, ammo, and rifles to see what actually works. I'm not sure anyone has actually tried it with an SKS, but it's possible someone might have.
Kind of a lot of trouble to go through if you just want a more quiet gun.