If you're not applying this cycle, you still have plenty of time to try to get to know another one of your profs. Or if you're desperate, just email some profs, ask them if they'd write you a LOR, tell them a little bit about yourself and include copies of transcripts/resumes. Worse case scenario is that they say no and you're no worse off than you are now. Best case scenario is that someone is nice and says they'll write you one (hopefully a decent or better one). That's what I had to do. I know I had two good LORs and I got a third one to satisfy some requirements (some schools require 3 science LORs).
I know it's awkward/weird to randomly ask profs but you want to be a dentist, right? You have to at least try (if it comes down to that). At least you'll have two good LORs. And don't forget to thank your profs!