cheap greensand in VA?

Greetings folks,
I am trying to garden very frugally, and I like the slow release rock-based fertilizers such as rock phosphate, greensand, etc.
Espoma products are readily available, but they seem a little expensive to me. Are there fiscal shortcuts I could take to find long-term fertilizer sources of phosphate and potash?
Does anyone know of a cheap way to get rock phosphate, rock potash, greensand, granite dust, etc. in Virginia (I live in Harrisonburg).
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