Originally Posted by oshusttoley
Do you recommend oem at dealer or going to like pepboys?
Personally on an 08 I'd get the appropriate battery from pepboys or equivalent and figure out how to register it to the car myself as mentioned above. But I'm inclined to jump in and do this stuff myself; I've nearly always replaced batteries myself prior to owning a BMW if possible, and intend to keep doing so now that I have a BMW.
If you're not inclined to get the hardware/software setup necessary to register the battery to the car, I'd call around to local independent BMW shops as well as the dealer and see who seems to be the best value. (always check out shops via yelp as well as the regional boards here)
Ideally an independent shop would be able to test the battery inexpensively to verify it is due for replacement.