Sorry, Zen, the clay fairy does deliver. All nice and pugged, in 25 lb. bags.
For stoneware, most people do buy it. It's fairly inexpensive, considering the labor required to purify it. A 25# bag is something like thirty to forty cents a pound. Bigger amounts go lower in price per pound. And the clay has been refined, so that there aren't large lumps or impurities, and has been run through a pug mill to eliminate air pockets.
The main problem would be finding the proper maturity for your clay. There are other clays added to a typical "mix" to balance out the clays so that they are consistent. They don't ususally consist of one local clay, at least as far as I know. Some clays are more "plastic", some are more crumbly, some have more of one attribute or another. So they are often mixed to give the potter the best of all worlds. Sometimes "grog" is added, which is already fired clay that has been ground into bits and sieved for size. Grog, because it is already fired, cuts down on thermal shock. In other words, since the grog was already fired, it won't shrink anymore so you cut down on the overall shrinkage of the pot. Grog is also decorative.
To find the maturity temp of your clay, you would have to refine it first, and then make little test tiles with it. Each tile would have to be fired at a different temperature, and then tested. Although, if you were going to pit fire, it woudn't matter because you wouldn't have that sort of control anyway.
If you spend fifteen hours, say, digging the clay and refining it... crushing it, sieving it, re-blending it, and wedging it, then you saved yourself about ten dollars. And in that twenty hours, you could have been learning so much about different clays, about using the wheel and firing... and of course, another ten or fifteen hours building your pit, buying the coal, etc...at least one full day of stoking the fire... that's just to test your clay mix and see if it has the attributes to work at all!
Oh, gotta go... the clay fairy just called and she's on her way. She drives a semi, you know.
And chews tobacco.
And her name is Hal.
Joanie