Due to the custom nature of what ever handles you might have made for you mule, there are not many production leather offerings for the Mule Team Knives...
But, there is one 'off the shelf' solution which has found some popularity with Mule Team knife owners... It has been said, that this sheath will fit the Mules even without handle scales, though the fit is loose. But, with a standard sort of handle scales, this sheath has been reported to fit pretty well.
Sharpshooter Sheaths makes what they call the Clovis Pouch for KnivesShipFree.com, http://www.knivesshipfree.com/Clovis-Pouch-KnifeFlashlight-Holster-Brown
Marion