Jamie
You can't buy profile 20" disc hubs in the same way you can a normal profile hub. As said above, you need to buy the internals of a 20" hub and then fit the MTB disc shell around the outside (may need a bit of filing) If you see a site that sells "disc profile rear hubs" CHECK THEY AREN'T 135mm!
If you manage to make one though, it'll be worth it, and it'll sell for loads on here too as I know lots of people, including me, are after one