Matt,
I had been waiting for a mech guy to answer but apparently there is no easy source for this item. You have probably concluded by now that a differential is a problem when used on a steering vehicle. Without some form of slippage in the mechanism, a turn will drag one wheel and stall the drive motor. I know if one GM team last year who fabricated their own and it was beautiful. However, two separate motors works just as well in our application and is easier to implement.
Hope this helps.
It should not be too much trouble to make a differential from stock bevel gears (for instance look in Boston Gear Catalog); however I agree that using two motors seems like a better solution. Our team has never used a diff, and probably never will. With two motors you will have twice the power, the speed can vary from wheel to wheel (the motors will do that automatically), and you save the weight and extra work of making a diff (building an automobile with two engines would be too costly).
If you are still serious about the diff, the gears could be mounted within a diff case made from four plates, bolted together. Some machining would be required, but not too complicated. The engineering section of the Boston Gear catalog is available on their website. It has info to help you choose the gear sizes, based on the speed and torque, which you can get from the motor data and your own gear reductions.