I bought a normal medium sized battery from Halfords, at the time it was £65. But I realise now I should have went and bought a one from the scrapyard at £15.
I got so sick of wasting money on leisure batteries and constantly trickle charging them, then forgetting to charge them, then they give up.
Any battery will do, your only running a controller and pump. Try and buy a relay kit from e-bay, and it will charge as your driving.
Good luck.