This is called a “thumb spica.”
Southwest Hand Therapy has them!
They’re near the corner of 35th & Alaska:
4621 35th Ave. SW Suite A
phone: 206-935-1215
I went crazy driving all over creation and ended up getting one way up north at Northwest Hospital’s outlet – can you believe it?! Later I found out I could have gotten one right here in West Seattle.
You may have insurance to cover this, but if not, I think if you have a prescription, there is no sales tax. I want to say they cost about $40.
Best wishes for a speedy recovery. And let us know how this works out.