Hi All ; It's been a 45 yr. search for me. I hope to shorten your suffering. I'll just give you the answer first, or the one that worked for me. Folate/Folic Acid. I need extra, more than the RDA of 400 mcg. , I need 800 mcg. daily in a supplement.
Canker sores are a common sign of B12 deficiency, so that's another supplement to consider, especially when one takes Folate, because they are interdependent. Sublinguals are easier for the body to use than oral types. I take large quantities, but I believe most people may need less. Supplements work best when we find our own numbers. My guess is that most people will respond to 500-1000 mcg. daily.
Zinc is another supplement that is suggested for canker sores. The RDA is approx. 15 mg. Daily. When I look at the zinc content of foods, it seems difficult to consume that amount, so I supplement with it, 15mg. 2-3 times weekly. But I'm considering a test just to see if I'm in the range somewhere.
Folate may also help with other lesions/ulcerations along the GI tract, I've read.
I wish I could give this information to my 10 yr. old self, when I lay crumpled on the bathroom floor with a stick of silver nitrate that only made the pain worse....