Since I used to work in the art supply business and sold miles and miles of rubilith back then, I'm amazed that it's so difficult to find these days.
We used to have several alternative materials that got used when the Ulano product ran out. Sadly, all of the companies that used to make it (Zipatone, Chartpak, Pantone and Letraset) have either gone out of business or changed their directions and product lines.
Casting about, I found this:
...doesn't have the adhesive backing but it's about as close as it gets. Also couldn't vouch for how photo opaque the red or amber might be but if the lack of adhesive isn't a problem, it might be worth a test. I suppose that it could always be sprayed with one of the 3M spray adhesives....