I am getting ready to start my first RMH and decided to buy the fireclay instead of mine it myself....even though I live in clay rich Ohio. I found the Hawthorn Bond 40 mesh for about $12.00 per 50 lbs from this place: http://www.ceramicsupplypittsburgh.com/
This actually helps me a lot! I also have plenty of clay, but I wanted something much more reliable. I am willing to spend a few bucks, to make sure that my house doesn't burn down. The price is very reasonable, in comparison to the value of my time/labor if I were to dig and process my own.