TV Jones makes a Gibson humbucker sized pickup...
http://www.stewmac.com/shop/Electronics,_pickups/Pickups:_Guitar,_electric/3/TV_Jones_Pickups.html
which I had considered for an Explorer, but they're just so butt-ugly compared to their regular pickups...
http://www.stewmac.com/shop/Electronics,_pickups/Pickups:_Guitar,_electric/1/TV_Jones_Pickups.html
I just couldn't bring myself to do it. If you use the Gretsch-sized pups, you'll very likely have to do the routing yourself.