Getting prototype parts made?

I am involved in the innovation of a product that in part will involve
getting a waterproof hydrodynamic housing made for a small water
propulsion unit (similar to the foot housing on a trolling motor). At
this point, I am in the prototype stage so was wanting advice on
getting quantities of one or two made as cheaply as possible. Beyond
that, I am open to suggestions on quantity runs etc. For the
prototype, I am only looking for basic structure and afterwards can
consider the materials properties etc. For starters, is it possible to
make my own parts from resins? Should I go with aluminum? Are there
other inexpensive ways to get a few parts made? Where should I start?

Thanks for all help!
Bailey--