Joe,
Any pet store sells stones but make sure you get a stone that has a plastic backing otherwise when the fish bang around in the well they WILL break the stones with out plastic back, I know from experience. I personally run two 10 inch stones in my well and it keep the fish happy. Here is a link to a picture of the stones from a local pet store

http://www.petco.com/product/9219/Aquatic-Gardens-Bubble-Bars.aspx

FIT