there are a ton of places that have performance bushing material... poly, delrin, etc.
A lot of folks have had good results with either E30 M3 or treehouse racing control arm bushings, stiffer motor mounts, stiffer subframe bushings, and the all-important reinforced rear shock mounts. BMWman91 has a poly diff bushing I think.
Anyway, replacing every bit of rubber under the car makes a huge difference.