That beveled plumbing gasket looks a lot like the rubber seal on an American Standard commercial urinal. Fits on a brass casting that's threaded to the tail pipe on the wall and bolts to the toilet.
Sorry for the unorthodox description! Aren't you glad to know this!
Urinal Repair - American Standard - Kohler - Plugs - Gaskets - Seals - Strainers - Spuds - Flanges - Screens
Scroll down to gasket #8138.
Any local commercial plumbing supply place will have one you can check out.
Larry