I used E46 cabrio mounts which I understand the M3s also use. They were about $32/pr from alloembmwparts.com. Made my own upper reinforcement plates from my old busted up E30 mounts by cutting the top off and enlarging the shock mount hole. Used RTV instead of a gasket by applying it directly to the mount and wiping on a thin layer of grease as a release agent to the body mounting point. Seals perfectly and next time you replace the mount the RTV separates from the body and remains stuck to the old mount.