I googled GrowStone and looked up retailers. Found one less than a mile from my office. Went there at lunchtime today and BAM! Got a bag (sure seems bigger than 1 cu. foot...) for $18.
They do sell it in one-gallon bags, as well as the large particle stuff for landscaping (1-1/12" rocks).
They also had Happy Frog soil conditioner, sold in a bag about the size of a bale of peatmoss. It must have weighed 50 lbs or more, for $30ish. I passed on that for now. Seemed like it would mostly fall thru a 1/8" screen when sifting. It says soil conditioner, so in your lawn it probably works wonders. Bark, bat guano, humic acid, dont know what else.
Here's the place:
Hydrogrow
12130 Nebel St,
Rockville MD 20852
240-669-8937