The Odyssey PC680, is a common replacement AGM type battery for the R65, and most other BMW bikes.
Prices can vary widely, so do some research before purchasing one.
I've had this battery since 2001, and my first one 'died' last August, and batteries in the Phoenix area live a hard life, a standard BMW lead-acid serviceable type battery would only last 18-24 months.
I installed a generic AGM type battery from a local company Gruber Power Services in my R65 last November, a co-worker has an independent BMW repair shop run out of his garage, and he has been using this particular battery for about 18 months and no problems as of yet, he has installed around 100 so far.