The later diffs are not an easy fit, it's more trouble than its worth,
E21 323i carry a 3.45:1 ratio and they are somewhat hard to come by. 320/6 and us 320i up to and including 79 run a 3.64:1 diff which is in my opinion the best performance ratio. Later 320i run a 3.90:1. (its really too short, causes 1st gear to be nearly useless.) 315 I'm told run a 4.11:1.
"iS" models and some non iS cars have limited slips. You can Identify a limited slip by a large "S" painted on the top or side and an "S" on the metal tag attached to one of the rear bolts.
Try this Al Taylor if you are in the southeast.
http://members.aol.com/atscparts/
Or Dave Varco if you are out west.
http://www.2002parts.com/
Either may even have a 3:45 limited slip for you.
Edit: OOPS, I see now you are in Australia. Cant' help with suppliers there.