A friend of mine has had a number of mechanical gizmos printed from Shapeways, and I would venture a guess and say you could get the shell of an engineered impeller 3-d printed, and then fill it with glass fiber and epoxy for less than 100$
I would try and cast my own impeller, by printing and cutting out templates for carving the lost wax casting, but that's a lot of work.
if the motor you have is larger than 5 hp i recommend a wound rotor, not magnets.