Is there a dental school in or around your city? If so, a lot of times you can volunteer to assist in the student clinics. The dental students are usually more than grateful for a helping hand, and you get to learn a lot too!
If there's no dental school near you, maybe you can volunteer at a dentist's office as an assistant (or if you do it as a part-time job, you can get paid). Many dentists don't require formal training (classes) for their dental assistants, so maybe you can find one that will train you him/herself. Its a great idea to get hands-on experience, so best of luck to you!