Hi Jason,
By "clean" and "unsprayed" I think you mean organic? If so, you might be able to find
local organic grain farmers willing to part with straw. Organic famers like to incorporate straw back into the soil, but in some parts of the country like the arid west there's a limit to how much organic matter can break down in a year given the lack of precipitation when the temperatures are just right for composting!
One of the book's contributors, Dennis LaGrande, was a rice and wheat farmer in the N. California Sacramento Valley, and he wrote a section on just how much fertilizers, herbicides, and pesticides are used on grain crops. Not much, at least in California, and usually applied early in the plant's growth so that there isn't any residue in the harvested material.
Ask around for organic straw, and if you can't find it, ask about how conventional crops are grown. If it's like California, it may not be ideal, but the straw isn't laced with nasty chemicals either. You might be able to work with that.