You can add another part to the found list, for possibly-perfect springs:
Amazon.com: Century Spring Corp C-570 Compression Spring, 1/8" OD (Pack of 5x6): Home Improvement
They seem to be the same size as the springs included with any pair of loupes, except for the length. You could probably cut them down to the proper length and then bend the ends of the springs to make them flat. The outer diameter is about the same as a loupe spring, allowing it to fit on the metal rod without it slipping off over the metal nut.
(BTW, I posted this in my own thread for *MY* ocular device.)