I don't know about online but if you're looking for cheap substrate, go to the pet store, and buy a bag of 1250 cubic inches of compressed aspen. It's normally used for hamster and mice cages. But for some reason, buying a small bag of substrate specifically intended for reptiles costs just as much if not more for a quarter the amount. I buy that stuff for 10 bucks a bag. And it should last you months and months; possibly even a year. Considering you're using it for one tank.
Thanks, I'll look next time I go.
Nate
1.1 Snow Corns
2.1 Hissing Cockroaches + A TON OF NYMPHS
0.0.1 Leopard Frog
0.1 Chinese Mantis