Yaris battery is kaput!

This week I have struggled with my stupid car battery. The alternator is charging fine but when I leave the car to stand for a while the battery is dead. I had the battery water checked and that is fine, so I reckon something is pulling current from the battery when standing for long.

Regardless it is now time for a new battery so could anyone advise on where I could pick up a new battery for cheap. Nothing fancy though, just the standard size battery is what I need. Where is the best place to shop?