You need to find a friend who lives in the suburbs LOL. I have a couple places up here to find different color granite along the side of the road, or even in a friend's backyard...
I do suggest that you find somewhere local so you can pick your own, it will be worth it. I have hunted through literally tons of rocks to find three or four that were just perfect for my 20 tall.
I have found petrified wood and slate at petland discounts up here... It might be worth a try. It is expensive, but how many pounds of rock could you need for a 8" tank?