Best deal I've found on battery is from www.getbmwparts.com
even with the shipping.... Plus they tell you what the model number is for the battery.
Locally I'm told there are a few shapes and sizes that will work in my car however I don't want a smaller capacity... nor do I plan on re-coding or want to have to figure out how to bolt down a different size battery plus the difference in price is negligible to non-existent. Finally the old battery can be taken anywhere for credit that further reduces price vs local.
One word of caution wrestling the super heavy battery is a beast and the odd cabling connections and number of cables doesn't make it any easier