I used to use Glyoxide for canker sores, too, and found it suddenly impossible to find. (I used to get the "larger" bottle.). Then I bit the inside of my cheek while eating pizza, and kept biting that spot. Ouch. Anyway, I used Colgate's Peroxyl, and it worked like a charm. It's a much larger bottle, available everywhere. I think you'll find it to be a very good substitute for the Glyoxide.
The pain they have cost us, the evils that never happened.