turner motor sport makes a good one, its the jim conforti chip....umm bavauto.com makes one i think. There a bunch on ebay.com but some can be a little iffy. Do a little searching and you will find some.
<img src='http://img127.imageshack.us/img127/4284/bmwlogo2new17jy.jpg' border='0' alt='user posted image' />
Black 4 door 1990 325i
15"x7.5 Ronal LS-wrapped in 205/50-15 Bridgestone Potenza G009