lipo from hong kong off ebay will get you going cheaply. I'm sure you can pickup a charge for about £30 also off there. non BRCA approved lipo packs will set you back like £20 each (sticks)
battery
http://cgi.ebay.co.uk/7-4v-5000mAh-3...item27ae8bfe2a
charger
http://cgi.ebay.co.uk/B6-7-4V-22-2V-...item414a68927c