Master cylinder for a 70 SS

I am in need of a correct (square) master clyinder for my 70 SS Chevelle, w/power disc's. Anybody knows of somebody who sells one at a decent price? The local parts stores seem to carry them but they are NOT the correct square ones for a 70 w/power disc brakes. Year one carry's them but they are quite Spendy! Ideas? Suggestions? The old one is gone!