So far I discovered that aspirin can be found in:
1. Willow trees (all kinds)
2. Birch trees
3. Alder trees
Wikipedia will mostly focus on willow trees and not mention the others and that is where they fall short on information. I recently discovered that alder trees also contain the salicin, which break down into aspirin, as well. I can taste the aspirin, and smell it (which is great).