You could also check to see if your school has pre-med/pre-dental advisors or committee members with existing contacts to dentists. Otherwise, I agree - open up the yellow pages, call the office and ask if they let predental students shadow practicing dentists. Then when you show up, wear something semi-professional/office casual.
Thoughts on shadowing for people new to the environment - do not use the word "blood", do not ask how many times they've done this procedure, try to avoid talking to patients if the Dentist is not there at the same time and comfortable with you speaking with patients, stay out of the way, show up on time, thank them for letting you shadow. Remember that in the end, you're likely to need a LOR from the Dentist.