You could probably use something like top soil. You would want to make sure it doesn't have any compost / manure or fertilizer additives or perlite (I've had problems with mine ingesting this from potted plants).

I like repti-bark and similar orchid bark type substrates. The chunks of wood are too large to be easily swallowed, it's durable and easy to spot clean. It also holds some humidity. I don't have the sheddding problems using it with my babies that I did with paper towels either.

-Alice