http://hobby-shop.qc.ca/plaskit_conversionUS.htm
has the International hood. And a GM TopKick one, too. Most aftermarket (resin) hoods use the Ford Shorthauler kit lizard mentioned as the basis, since the cab is pretty compatible with the other makes of hoods. You can also scratchbuild it, if you're not in a hurry.
For the Ford, a lot of modelers are using the diecast Ford Cruzer assembled model as a basis, but the Shorthauler is often mated with emergency vehicle bodies.
For crew cabs, you're on your own, although most look like two cabs spliced together anyways - like the International, where the back door is cut at an angle as if to fit around the windshield.
Another source for models you can modify is
http://www.toysbydg.com/catalog.asp?sesid=200410114387011136880792
I've heard they're pretty good outside, nothing inside, but the price is good compared to some resin stuff.