I don't know if there is such a thing. I've been looking myself. You can get new ones from http://www.rottendog.us/mpu004.htm They are $250 + $7.75 shipping + a processor if needed + ROM if yours isn't fried.
You can also email Steve at Florida Pinball Salvage. His email is Stevepinfla *at* aol *dot* com. His prices aren't bad but don't expect it to be cheap.