Many online retailers carry them - drs foster and smith, marine depot, etc.
Alternately, you can use bits of live rock rubble (cheap to get at most LFS) or make your own out of portland cement and aragonite sand. If you make your own, you can use disposable paper cups as molds - just mix sand and portland cement 50/50, then add water until it's like runny oatmeal, and put a spoonful in each paper cup. Let them cure for a week or so, then put them in a mesh bag and let them soak in freshwater for a few weeks (many people toss them in their toilet tanks). Then, one more rinse and you're ready to go.