I have a new 4-clutch 3.64 LSD from Metrix Motorsports on it's way to me in the next few days. The diff is for my race car build for BMWCCA club racing (99 M3 for I-Prepared).
Metrix has the lowest prices by far and their customer service has been outstanding thus far. I paid about 50% less than what it would have cost me from the other diff vendors.
Metrix is rather new to the scene and working on building their customer base. There are a few club racers using their diffs right now and a few running them on street cars as well.
You can email them at email@example.com
or call them at 818-545-0866. Ask for Jeff.