I know that UC Berkeley has an online program. I believe it's called UC Berkeley Extension and I know they offer tons of classes. In general, look for continuing education type programs. That way you can enroll in a class without having to go through the whole admissions process for that school.
However, with that being said, I would call the dental schools you are applying to and ask if online credit is ok. I was thinking about taking an online class as well when I applied last cycle, but when I called UoP they said that they did not accept online credit. So, you might want to call just in case!! Hope that helps!