Everybody has them for sale, but what I do is have my original castings/pumps redone by Skip - he has his own special impellers for the 37-48 pumps and does really good work. His pumps have really helped the cooling on my very high performance 42 Merc engine. Plus, he is a good guy - stands by his products/word and does great coil rebuilding to boot.
http://www.fordcollector.com/water_pumps.htm