What clear plastic stripping is used to wrap a toroidal power transformer?
Ok, I think I pieced the answer together from the comments I received, I'll lay out what I found here for anyone else that may find this on search.

I'm very confident the material is Amide-modified Mylar heat-shrinking tape, commonly used for inter-winding insulation and outer covering of toroidal transformers.



I found a supplier on eBay, which was the only supplier I could find anywhere, although I gave up looking after about 20 minutes :)

The pictured item is 40 yards of 3/4" width, 5 mil thick, generally used on toroids somewhat bigger than the one I'm working on at the moment. Here's the link to that item in case anyone is curious.

According to DuPont, the dielectric strength of (their) mylar is about 4kV/mil at this thickness, so half-lapped and double wound as mine was, it provides a heck of an insulating layer, almost 50kV nominally.

I ended up reusing what I unwound from mine, here's how it turned out:



I ran over it with the heat gun afterward and it tightened right up the few edges that were not quite flat. I can't tell the difference between this one and the others I have that are factory fresh. Except for the yellow mylar tape which I went a little crazy with; I was worried it wouldn't hold when I shrunk it again but that didn't seem to be any problem at all. I'll probably pull most of that off before I wind the secondaries; might as well keep it looking professional :)

Apparently the way you cure this stuff is by putting it in the oven at 320 degrees F for an hour. I have a thermal fuse wound in that trips at 105C so wasn't sure I wanted to do that :)

Anyway, that's what I found out. Hope that's a help to someone. Many thanks to @PlasmaHH and @WhatRoughBeast and others for their helpful comments :)