Here..
http://www.turbostart.com/search.aspx?q=sr59
As far as I know,they are the only ones who make/sell these.No other retailers sell them yet,as they just came out a few months ago.
I highly recommend them,as I just picked up two a few months ago.
Understand these are the new Axion owned 55lb. solid cell batteries,not just the mock casing with the smaller ATV battery inside.