I’ve used the sites above to create gears cut from wood and Delrin. Faster than 3D printing, but if preferred, take the resultant SVG, import into tinkercad, SketchUp, etc., and extrude to desired thickness. Export STL and you’re ready to go.
Another place to check is McMaster-Carr’s online catalog. They have 3D design files in various formats for pretty much all their extensive hardware catalog. Although I have to say, that printing them still takes some adjustment. Maybe because I’ve used resin, not pla, but parts I’ve printed from the files never seem to fit perfectly with the thing they are supposed to mate with.