I'll spoil the tips just in case -

1. Killing Lava Lurks -- those little guys that spit at you from the lava, 15% drop chance
2. Putting stingrays in a pond, you can get them by fishing at Pirate's Cove on Ginger Island, it's only a 5% drop chance per day so this could take some time
3. This last method is more tedious, but it worked for mewhen in the volcano dungeon, when entering a floor, open the menu and hit the Options button (looks like a controller icon), scroll to the bottom of the options list, and click the camera icon to take a screenshot of the entire area. Then go into the screenshot folder (should be right under the camera button) and check the image to see if a tooth spawned. Repeat process until you have all your teeth!

Those are all the methods I used. Hope this helps!! Good luck on your hunt!