I usually take rocks from outside because buying rocks is like buying water. You can also get them from the hardware store. I wouldn't pay the ridiculous prices for the petstore rocks...they're the same rocks just marked up because it's a pet store.
As for what type...well basically anything. Africa is very geologically diverse. The easiest to come by would probably be granite, limestone, sandstone, and greenstone. Of those, greenstone is probably the nicest for an aquarium, followed by sandstone.
If you're into geology, read this.
http://www.fortunecity.com/oasis/ske...94/geology.htm