Shop around... you will find them everywhere from Kawasaki to K-Mart and most auto parts houses in between. Motorcycle service is tough on batteries and I seem to have to replace them every couple of years. Even still, I have no idea what the best brand may be (assuming that there not all made by the same company anyway). That may be why I have to replace them so often too<img src=http://www.ezboard.com/intl/aenglish/images/emoticons/frown.gif ALT=""> I suspect use and climate have the most to do with how long they last.
I`ve had real good luck with the OEM,(Yuasa ?)my ZRX had over 3 years on its battery & was still going strong and you never have to add water and Arizona is hell on batteries.Might want to make sure your bike is charging,sounds too soon for a bad battery.
I went to the dealer and they only gave a 6 month warranty on the Yusa battery. So I checked around and Sears Diehard was the only one with a 12 month warranty. So I have been buying them ever since. I think I went through the original Yusa battery in 3 years and I think the Sears Diehard lasted just about the same though. 12 years and 67,000 miles and I was on my 2nd Diehard when I sold the bike. It was an 89 750 Ninja.
I have given up on Your assa and the usual lead acid battery. Nothing bums me more than maintenance and a vehicle that won't work.
The 98 BMW is running a Westco AGM battery. as is the '01 Conc is running a non-maintenance as well, some other brand.
Try www.batteryweb.com. They sell 3 different types of AGM maintenace free batteries (gel batteries). Prices are reasonable.