Pebble, or river rocks, are very common in aquariums. I have some in my tanks as well, and they are cheap, have nice natural colors, white or black, and easier to take care off than the smaller gravel. You can find them in craft stores, Wallmart, or even HomeDepot in the garden department. In the craft stores, I find them in the isle where you can buy supplies for glass decor. You know, like filling up glass bowls with rocks, or even decorate around candles, or the isle with the sand art.
Lets see if I can find a picture of a package.

Got it!
http://www.michaels.com/art/online/display...oductNum=fl0057