Darrel's Economy Muffler is a good choice if you are trying to keep it inexpensive. kinda depends on the level you are looking for- IMO Bassani tends to be the best fit and you can do that yourself
I have been really happy with the work I've had done at Performance Discount Muffler, on Broadway around 27th. Rob is quick, always does exactly what I ask for and his prices are very good.
The only down side is he doesn't work with stainless. Anybody know a Portland area shop that does? I'm willing to pay some extra bucks to fab up an H-pipe out of stainless.
I had A Better Sound weld some cats in, and they did a decent job. Not show quality welds, but they were cheap, fast, and easy to deal with. They're out south, I guess more or less near Oregon City (?).