Hmm, I dont know about gravel, but if you want to use black sand
you could get it from what they use for sandblasters, Im pretty sure, with proper cleaning. And its in bulk, so that means plenty of sand for less.
I have some glass grit sand in one of my tanks and it has been working fine.
Im not 100% on this but im pretty sure it works, maybe someone can back me up.
hope that helps.