This question is asked a lot and if you do a search...you will find that the Duralast 49D (approx. $69.99 sold by AutoZone) is usually the best deal. The Interstate Battery and the Douglas battery (sold by Batteries Plus) can also be found usually for under $125.
Here's one of many links that's been posted: http://bimmer.roadfly.org/bmw/forums/e34/5973423-1.html
Oh yeah...DON'T forget to get the vent kit...it should be attached to the side of the battery. It is VERY important to vent your battery if it's enclosed under the rear seat.