I got a big ass roll of MLV from an insulation & fireplace materials wholesaler in Houston (where I live). They carried rolls in 2-3 different densities. http://www.perfectionwholesale.com/sound-control.html
I'm sure you can find similar companies that carry the stuff in most major cities. MLV isn't really an auto-specific material. People have been using it in building walls for sound proofing for a while.
It wasn't cheap, but it cost less than getting it from SDS, plus I didn't have to pay to ship a 100+ lb. roll of material halfway across the country. I got all my other materials from SDS. I got the stuff for my wife's car, but I ordered extra material to do my taco after I finish with her ride.
EDIT: This is a company that actually makes the MLV... and a list of their distributor locations across the country: http://www.soundisolationcompany.com.../distributors/