Where do I go to get a sore (probably canker) sampled and analyzed? . I’ve had visual examinations but I want to know for sure what's causing this.

Canker sore. You may have what is called an aphthous ulcer or canker sore. This is small sore that appear on the inside of the mouth, the cheeks, the inside of the lips, the tongue and anywhere on the gum. Canker sore is the most common oral lesions. Individual ulcers typically last 1-2 weeks.