You can buy it in gallon jugs like at Walmart but how big of a tank do you have and what sort of lights are you running? If you have to top-off a lot of water each week, it would be wise to invest in an RO/DI system. They don't cost that much off a certain auction site!
If you buy it at Walmart, make sure it is purified water and not just spring water. I think if you look on the label, you will find something like purified using reverse osmosis or something like that.