I have stopped looking for special doll length zippers long ago after I bought a dozen of special doll zippers and none of them worked simply because they didn’t have the top stoppers and the sliders were sliding off.
So I have switched to regular zippers we can find at any fabric store.
I do prefer to buy 4” long zipper for Disney Animators and Little Darling dolls, and 7” zippers for American Girl dolls and my other 18” dolls. But if I don’t have a short zipper in the color I need, I do use 7” or even longer zippers
Actually it’s quite easy to shorten a longer zipper to the exact length you need. For the pics below I used an unfortunate doll size zipper that had lost its slide.
You will also need a sewing awl and small (jewelry) pliers.
1.Using a small awl or any other sharp tool with a fine tip and working on the wrong side of the zipper pull the teeth of the bottom stop out of the zipper body, straighten them out, pull the bottom stop out of the zipper body.
2.Reinsert the bottom stop back into the zipper body at the desired level. Push zipper teeth into the body and crimp them tightly to secure the stopper in its new place.
3.Clip off the bottom part of the zipper to the desired length and use Fray Check to prevent the cut from unraveling.
4. If all of the above is too much work, you can secure the zipper teeth at the level you need with several tight stitches (using a regular sewing needle and thread ), add a drop of Fray Check or some fabric glue to secure and stiffen stitches and cut the rest of the zipper, including the bottom stopper, off.
Less work but almost same result!