Well, there was Dusty Johnson (pvsaddleshop.com), who was an expert with leather, but he's no longer with us, unfortunately.
I had mine made by Vintage Motorcars, see https://www.vintagemotorcarsin.../shift-boot-leather/ (and https://www.vintagemotorcarsin.../leather-brake-boot/ ). They have a few colors on hand, they sent a few samples ahead so that I could see in person, and thankfully I lucked out because one of them matched very nicely. And they can do a custom stitch color. I think they can get more custom than that, but I don't remember the specifics.
Vintage Speedsters vintagespeedsters.com also does them, both leather or vinyl if you're okay with that and want to save on $.
Beck beckspeedster.com also does them, I think they offer the most custom options if I remember correctly.
That's my knowledge base on this, and the info is about a year old circa mid-2020, during the pandemic that shall not be named.