your best bet would be to make your own. the only manufacturers that i know of that make silencers are Apexi, Comptech, and Tanabe (not too sure about this one). The Apexi and Tanabe are both for the 4" tip and the comptech one is for their 3.5" tip.

all a silencer is from what i have seen is a straight pipe about 2" in diameter, welded to a can lid that will fit inside the tip. I was going to try and custom make one for my DC Sports, but figured it would be eaiser to just sell my exhaust and get an Apexi WS. the design was pretty simple. make a canister out of some metal and put two ends on it, fill it with some noise absorption materials (baffling material used in motorcycle mufflers), and put a pipe through it.