Slate can be had as floor tile in 1 ft squares at the home improvement center for fairly cheep compared to slate at the LFS. The stuff I got is about 3/8 inch thick and can be broken into the shapes you want. I have used pieces to mount sticks for the driftwood look and have built fish caves out of it using aquarium silicone to glue the pieces together in the shapes that I wanted. Just be sure you let the silicone cure until there is no more odor and it will be safe for the fish.
Just beware.... Many natural rock tiles are pre-treated/sealed. Do a bit of research before placing that stuff in your aquarium.