You can't beat getting something FOC .
I was a bit gutted at having to spend £100 on a battery for our Fourtrak. My neighbour's off to france for a few weeks soon so it's better to find it's dud now I suppose . Still a shedload of money for a battery though .
There are plenty of good, high power leisure batts around for a lot less than that. Even a real traction/deep cycle battery with a decent AH rating shouldn't be more.
I got 420AH's worth of ex UPS batteries off an ebay seller for under £100, including delivery, and they are in top condition. The same seller seems to have a line on them and they come up regularly, many different sizes, and i'll bet they're all decent.
For vehicle batteries, Fleet Factors - lorry factor co, do good prices. They don't stock the smaller sizes, but for 4x4's etc, they have the lot. Mind, the prices have nearly doubled in the last couple of years (hilux battery was £30, now over £50)