There are a number of places to get one. The first battery I put in mine was from Motobat. That was a good battery and lasted a little over three years. When I replaced it I needed one in a hurry so I just went to WalMart and picked one up of there's. I've had it in the bike a little over 6 months and it has performed well so far.
...and yet my Motobatt is still going strong after nearly 4 years. My battery was one ordered of four (for friends as well as me). The only one that failed, oh a few weeks ago, was one my friend bought (we were in AZ at the time, my motorcycle sat over the winter this past winter, otherwise I rode it year round) for his GoldWing and it sat for last year and a half with no real use and he just told me it failed. Go figure.
I bought a new Scorpion battery recently from batterystuff.com. The cost was $55 with free shipping, ordered Sunday night, had it on Wednesday(from Oregon to Michigan). I haven't had it long enough to say anything about its longevity, but , so far it's starting my 650 quicker than the Motobatt it replaced ever did. The Motobatt died after 22 months in my case. I would recommend putting any battery you buy on a charger before use to top it off. They say ready to install and use, but, you don't know how long they sat on a shelf before you got it. My two cents worth. FWIW.
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