They are available from Gripper , but its more than just the diff thats needed . They also need one of the driveshafts from the car (cant remember which side) as they have to change the inner coupling . When I enquired the cost would be more like £1.5k all in.
Just out of curiosity , are you intending to take it rallying ? Imo the Willy gets its power down superbly and the only real advantage you would get would be on very tight 1st gear turns or on gravel , unless you also do a lot of track days . If its just for road use I would probably put the money elsewhere or do you have in excess of 200bhp ?