I came here to say exactly the same thing. I'd love to find a kit or gearbox than can gear up/down a simple stepper motor for Arduino projects. The HDPE laser cut is a good idea though, I'll give that a try.
Inkscape has a plugin for gear teeth pitches. I no longer have the link to it (or the install because it was 13 years ago), but I was able to cut a whole bunch of HDPE gears. THe next hard part was getting hubs to match with them. :|