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.
In the DC Metro Area? Check out GWAPA