The zip on my jacket broke, but fortunately, I had recently finished building a 3D printer! So, like any good 3D printerer, I took the broken zip parts, measured them up, modelled them in Solidworks and printed a new one! It works great, and prints in less time than it would take me to drive into town, buy a new zip and come back!
(Sorry for the Solidworks model, I'm planning on getting familiar with free 3D CAD packages so that anyone can modify the design.)
There are two files to download: Zipper and Zipper with small slot. I've been using the Zipper.stl file, but as requested, I've uploaded a version with a smaller slot size in order to increase the amount of plastic around the slot for strength. So, if you're having trouble with the slot breaking, please try the Zipper with small slot file.
You'll probably need to print this with a brim as the half of the slider without the handle has a small surface area and can get knocked off. I've used a 5mm brim myself.
I've printed and tested this using a 0.2mm layer height and it works great.