Pre-health advisors are a great place to start. They can recommend volunteer opportunities available through universities and give you an idea of the time commitment those projects will take. For example, University of California, San Diego offers a list of volunteer opportunities on their pre-health advising webpage.
Joining premed or service clubs is also a great way to hear about new volunteer opportunities, clinical and research experiences, and more. You can also check out the AAMC’s recommendations for volunteering.