www.drexler-motorsport.com have one (up to 100% locking just like the current M3), one of my friends has a 130i with this limited slip diff. Great sliding and perpetual sideways motion(AND you have TRACTION)
BUT the 130i's gearing and differential differs a bit from the 330i's gearing and diff.
Prices vary but for about 1,900 EUROS you get your LSD
and then they'll take your 'old' open diff back.