Hi Will
We have seen two different designs of replacement GameCube 3D modules, one which is a copy of the original and another with a different design.
We have been looking at the Game Cube 3D modules ourselves for our third party controller in order to improve it for Smash bros and cannot find a source for OEM which were made by Mitsumi. To buy from such a manufacturer would require massive minimum orders like 100K pieces or maybe more.
I believe the issue with the third party sticks is either the design used or the plastic quality, the part is actually really basic and just a few bits of moulded plasic in a metal casing and two sensors on the sides.
The issue seems to be with the middle plastic parts wearing and failing, the plastic grinds down to a white powder.
Could you post a picture of the top of the thumbstick to down what the inside looks like, i could probably tell you if you used the version that is a copy or alternative design, it might be worth you trying the copy style as it might last longer.
We are hoping to produce a better 3d stick for our GameCube controller this year but it will be later in the year.
Kieran