The easiest is to start by asking your own dentist. If he/she doesn't want to then just ask them if they know any other dentists that would be willing to let you shadow. That's how I got my shadowing in. Went to my own dentist to shadow for a few weeks and then he referred me to another dentist, did another few weeks there, and then he referred me to several specialists and a dental lab.....so it all goes from there.
If not, you can randomly call up dentists' offices and ask around.