I build synthesizers on the MIDIbox platform. I've completed three synths, and am working on my fourth. The first two used semitransparent 3D printed plastic button caps and "light pipes" for LEDs (you can see them in the picture at the link above). I found the maker on this forum about three years ago who I used for my first two synths, but I haven't been able to contact them recently, so I figured I'd ask for someone new. I have a couple friends with 3D printers, but nobody with the transparent filament I need.
The parts are quite small, max dimension about 12 mm for the buttons, but I will need a couple hundred of each kind, so the order would still be moderately priced ($40-$80 range). Single material, any transparent plastic (the slightly bluish ABS pictured above works fine). I will provide STLs and work with you on getting the details right before the whole batch production (the buttons have a recess in the underside which may cause inconsistencies in printing, depending on your machine's capabilities). If the batch I receive works in my build, I also plan to sell these parts on the MIDIbox Shop in a kit for the synth's front panel, so a larger order would be on the horizon.
Let me know if you're interested. Reply, PM, or email me at my name at Google's email.