Those holsters you pointed to aren't lined, which is one thing you could offer that most mass produced holsters don't offer. Most knife sheaths aren't lined either which is doubly bad for high carbon blades. Doesn't take much to make a wood, kydex or even plastic liner for the inside of the sheath. There's two things you could offer that would be one up on the mass produced versions. My local cobblers are always getting request for custom sheaths and holsters, I'll bet the cobblers in your area would also refer these clients to you because most cobblers just don't have the time to make custom stuff even if they wanted to.
I've been making small things out of oil tanned leather, which saves me from having to buy dye and scrap wool to finish veg-tan leather; also saves me from having to buy stamp tools, but the small things I make are actually paying for all my leather and allowing me to collect the tools I need for carving and dying veg-tan.