While most of their rock is for commercial type applications, they do have a large selection of lava rock and river rocks (or river tortillas as they call them). The river rocks come in an assortment of sizes and colors. I'm sure there are others that can be used in tanks too. One word of advice though, is take your own bucket or sack if you are getting a small amount of rocks or they will charge you for the sack that they furnish.
They've also been around for a long time and have a great reputation.