a good place to check out universal muffler is
BigExhaust.com.
Magnaflow has some dual outlet mufflers that should weld right in. You just have to remember that these are straight-thru design mufflers, so they will be louder than your stock muffler. If you have to replace the B pipe (or whatever the pipe between the cat and the muffler is called by you Bimmer guys
), get a very long resonator installed to reduce exhaust noise.