The "original" batteries in my '07 530xi with CA lasted about 3 years (production date Aug '06) and were replaced by my dealer under warranty. They were working fine when I traded it (December 2012) for my 2013 535xDrive.
Replacements are available in places like Radio Shack (the most expensive), big-box stores (mid-priced) and believe it or not - supermarkets -- Always on sale and since a few of my remote devices use them I buy then when they're on sale. Picked up two 2 battery pack of a name brand (Duracell I think) for $2.27! Such a deal :-)
My opinions are free - you'll get exactly what you pay for!
1998 Z3 2.8
2007 Rav4 V-6 AWD