Something like this is a good option ..
Any 12v 30+ AH AGM style battery will do the trick. You can get AGM (gell) batteries form lawn mower sized all the way up to RV size.
Don't buy a wet sell lawnmower/motorcycle battery. They do not handle cycling well at all, are a mess to deal with, and overall are crappy batteries.
Unless you live out in the woods there is probably a battery store near you. Go there.