Give any auto parts dealer the group number of the battery, & they can match it. They may not stock the battery, but they can get it for you in 24 hours. I got my last one from O'Reillys, for 125.00. I buy quite a few parts from them, & negotiated the price down from list price. I would not worry if it is a lead cell or AGM style of battery. You will get a 3 year non prorated warranty, with the final 3 years prorated. BMW only give you 2 years total.
The battery must be registered to the car. None of the auto parts stores can do this. They will install the battery for free, but they can not register the battery to the car.
Hope this helps!