Ben Massell dental clinic. They're a volunteer based dental clinic designed to help the impoverished. They're usually very good with letting students shadow, esp if you're from GaTech.
I was also able to get a yes from the Children's Dental Care of Atlanta. They're affiliated with the Egleston hospital system of Emory. I had to talk to one of their dentists upfront thou.
You'll get your hours done from these two places, but you may not be able to get a rec letter from it, since they rotate dentists often. If you want a letter, you'll just have to go to more clinics and be willing to face the no's. If you find one, let me know because I had to get one outside of Atlanta for it.