Lowes and Home Depot carry something called Schultz Aquatic Soil. It's pricier, because it only comes in small bags, but it's the same exact stuff, just a light brown in color instead of hte darker Turface color.
aquariumplants.com also sells a substrate, which again is the same exact stuff, just under a different name. Your best bet price-wise is to find locally someone that carries turface, like lesco.com, but those are some other options if you're not able to do that.