Where can you buy diatomaceous earth?

I am battling green water and am focusing on getting my ferts and CO2 balanced, but I would also like to run a diatom filter to pull the algae out and help the plants regain control of the tank. I have the diatom filter but it doesn't seem to help that much, so I would like to add some diatomaceous earth. Do fish stores sell this? Where can I get some?

I don't know where the nearest pool supply store is, so I'll check the hardware stores. Thanks.