I have the purchase information from the person who actually made the original strap. She is a horn player and makes most of them for horn, naturally, but she will still make them for euphonium. Start here:
http://comfyhornstrap.blogspot.com/
Scroll down the page until you see this picture:
Here email is right there, with some words replacing the symbols @ and the dot. Please do not post the email here so we respect her right to not get it scraped by robots. It's her personal address. This is an out-of-the-home business run by a horn player who is good at sewing also.
I'm going to order a new one for myself - she has added a fleece lining for comfort. Standard black is $29; the one in the photo above is $33; and she will do custom for a higher price.
[Update 12/2022] And here are some from my current collection:
I did a video about the hand strap. I included it in this post later in the thread:
http://www.dwerden.com/forum/showthr...829#post152829