It sounds like you are using the same motors that I am using. I picked up a four axis driver from this place:
I have a few more weeks till I get my machine to the point where I can try it out so I cant offer any of my experiences with it. I only paid $110 USD for mine. Has anyone else tried this driver that could tell us if it performs well? This board is set-up for 4-wire motors, but you can just wire your 8-wire steppers in parallel or series. Good luck!