Probably the most affordable is the Bursch 912E "quiet" system. Thought I saw it here on Pelican, but GPR sells them out of SLO, CA. Also, I saw some 912E exhaust stuff (incl. thermal reactors!) for sale in the SF area at:
http://www.pca-ggr.org/nugget/nugget97_01.html (don't know how old the ad is.)
Now for the fun part. When I had my 912E (which,luckily still had the stock muffler) I met another owner who modified an early 911 dual inlet/single outlet factory muffler to mate with the typeIV/2.0 flanges. Didn't seem like a heavy project (if you can weld) and the finished product hung with factory hardware & looked absolutely stock. Like the stock factory 912E, the sound was reasonably quiet & there are a number of good factory 911 mufflers around for about $50. The factory units either rust or don't rust. If they haven't rusted, they last forever.