Polaris never has had good batteries.  I had pretty good luck with interstate batteries but now I've heard they are offshore.  I'd try a Deka and for sure get the gel sealed battery.  If you can squeeze it in there a Odyssey PC925L is a great battery.  I put those in the ranger and rzr.  Battery mart is where I've been buying them from, for about $145.  Double check the dimensions but I think they fit.