They are often used in aquariums try a pet shop.

Depending on what size you're looking for--try a craft store (AC Moore, JoAnn fabric, Rag Shop) for bags of small rocks--they go on sale all the time for 99 cents.

Larger (palm size) can be bought by the bag--about 20-25/bag--for 10.00 at landscape shops here in NY. They're called exotic Mexican stones.

Try the Stoneyard in Littleton, out by 495. I think they have them listed on the website, including lots of variations, colors, and sizes.

Here is a link that might be useful: Stoneyard pebbles