Off the top of my head ... Bison, Bodine, Groschopp, and Baldor. Those would be my bet on who might have a motor like this, although I haven't actually checked if they do.
A 12V, 1/2 horsepower motor is actually a tough design because you have relatively high current draw which means larger diameter wire which means it is difficult to wind. If you can change to 24V, it will probably make your search easier.