I had the day off today, so I went around looking for a muffler. I ended up buying one from a local muffler shop. The guy came out and listened to my Borla, then said a Flowmaster 50 series would probably be just as loud. They had a turbo style muffler that that he thought would be quieter, so I ended up getting that. The guy measured my pipes and sold me the muffler, adapter pipe, and clamps all for just over $30.
I brought it home, cut off the Borla, and put the turbo muffler on. It sounds a lot quieter at idle, and under acceleration, but it has a really loud drone at 2000 rpm -- louder than the Borla

The drone is right at highway driving speed, so I'm sure it would drive me nuts. Actually, this $30 muffler has a nicer tone than the Borla. It's a nice deep rumble.
I guess the good news is that everything is just clamped on, so I can take it off and throw something else on there quickly. I couldn't find any shop in town that sold the Walker or Thermal Research. I guess I'll have to check mail order places. With all of these mufflers, I may have to open my own performance muffler shop by the time I'm through!