IMO, sand is the only proper substrate. You can get just normal aragonite playground sand, and use that as substrate. There's a certain brand people really recommend on this website, but i can't remember the name.
The two types of playground sand is silica and aragonite. Silica is generally bad for fish tanks, although it can be used. The texture of the silica sand allows algae to multiply on it more rapidly, making it more of a nuisance.
A good way to test if the sand is aragonite or silica is by simply using vinegar. You put a small amount of vinegar on the sand. If it bubbles, that means it's aragonite (calcium-based). If it has no reaction, that means it's silica.