I would recommend a deep cycle leisure battery of about 85AH, this will give you about 2 full days work between charges. You can also wire this up with a split charge relay (I use this on one of our vehicles) which will allow it to charge from your vehicle's alternator. If you choose the split charge relay option you might find on the odd occasion that at the end of the day if you have not done much driving that the pump runs slow, if this happens then simply start the car's engine.
You can buy these batteries from most motor factors, find a local one and the price will be about £50-70.
Another way of powering these pumps that I use (on one of our other vehicles)is to simply run it off of your vehicles own battery. This is a very simple way of setting up. You will need to check that you have the largest battery that your vehicle can fit, if you have a large diesel engined vehicle then you will probably be OK. I find that this works very well although you will reduce the battery life from about 3 years down to about 18 months. As soon as you suspect that the battery is getting old, buy a replacement one and you will never be caught out with a flat battery. To get a feed simply run a wire and fused switch off of the + terminal to the pump and then hook up with a good earth connection near the pump (the rear light clusters often have a good earth source).