The C6 has a built in bellhousing so you are limited to getting one from either a truck or car that had a 460 or MEL family block in it from the factory. The heavy duty ones have ribbs on the top of the casing right where the bellhousing mates to the transmission so that might be something to look for. Your best chances is to get one out of a truck most likely but all are pretty stout transmissions to begin with.
Kevin