A Humber Mk2 what is the first question. Armoured Car, Scout Car, Light Reconnaissance Car or something Soft skin.
Most of the Humber transmission's have cross over's into other Roots Group Vehicles which may supply components or be a direct replacement. Either way you will need to find someone willing to sell you one and transport it to the states. Don't expect to buy any gear boxes off the shelf anywhere. There is a very good Humber FB Group.