Depending on the quality you'll be happy with, I agree that buying the holster outright might be a better deal in terms of price and quality.
There are faux leathers without the white backing - but even if you used that you could still line it - with something like black faux suede, even. Since nobody can really see in the boots/straps that shouldn't be an issue - you can either make the belts basically a tube or just fold edges under. Since top-stitching on leather is common and a good look, I'd probably just cut a piece twice as wide as the belt and fold it so that the edges butt up against each other on the back side and top-stitch down the sides so it stays like that. If the faux leather doesn't have enough body, you can sandwich something else in there, even craft foam (probably a good idea on the tops of the boot covers so they don't fall down under their own weight).
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan