Irwin Stone in Rockville/ Frederick carries a large variety and are not really expansive. Ask them to see their discards as they will usually give these to you for free.
They also carry Pumice, which is great of you are doing a lot of rocks, because it barely weighs anything, and you can carve it with a screwdriver into any shape you want.
I will be giving a talk on this at the Feb. CCA meeting if you are interested in trying it hands-on before buying it.