Differential Buying/Building

I’ve searched for ready-made differentials for robots, but have come up empty. Does any one have any suggestions on any possible sources?

Failing buying, then I guess I’m into building my own. Does anyone have anything on how to do this as easily as possible?

Look here:
Under the “Miscellaneous” category.

Granted, it’s $300 for 30 oz-in of capacity, but it IS a differential.