What is this sanding disc and where could I find one?
That looks exactly like a Porter Cable paint remover disk, at a glance. 24, 36 or 48 grit, $20 bucks or so.



The tool it's made for: the Porter-cable 7403 abrasive paint remover. There probably is a backing pad that supports the disc.



Here's the pad:


Not affiliated, don't own one, do own some other Porter-Cable tools, happened to notice these discs when looking for some sanding supplies once.