Creeks, Streams, Rivers is usually where i get mine. I have access to alot of slate also. But whatever rock you want to put in your aquarium, you should do an acid test to make sure it is inert. this can be done by scratching the surface of the rock and dropping a drop of acid such as Muradic or hydrocloric on the scratched area, If it fizzes it is no good! and will messup your water! always boil them before you put them in, with the exception of slate, It can be rinsed off and ready to go!