Hello Melissa: I recommend you check with the following resources on where you could volunteer as a Lab Assistant in a clinical setting. Check with your:
1) Guidance Counselor to see if he or she is aware of any volunteer opportunities
2) Local hospital, clinic or medical facility. Let the Administrator know of your career interests, etc.
3) Family doctor. Doctors usually have a network of colleagues in other disciplines; so make sure you inquire
4) Parent's family & friends, co-workers, etc. You definitely do not want to overlook here because you never know who knows who; and, where an opportunity might turn up from.
Here are a few sites I researched to help you get started.
Good Luck to You!