Hello!
Unfortunately I don't think there is one site that will set you up with a PA to shadow or a volunteer opportunity. I wish it was that easy! And if it exists and I don't know about it..please share!
Finding a PA to shadow can be really tough especially with the HIPPA laws, etc. Do you know any RNs, NPs, MDs, DOs, PAs...anyone in healthcare?? If so, you might want to start there and ask them if they would not mind having you shadow them. Even if they are not a PA it will get your foot in the door. I started off shadowing a RN-FA in surgery, and ended up getting to know multiple MDs and PAs in the surgery room when I was shadowing! It was great because they set me up with other connections, or let me come in to shadow them at another time, etc. You can also try to just cold call/email any PA in your area. You probably won't hear back from many of them but all you need is one to respond and allow you to come in and shadow! Finally, I would suggest volunteering in a hospital or finding some kind of clinical volunteer internship where you work in a setting that there are PAs working around you. You can meet them there, and ask if you can come back at another time to shadow them. This way, you are already an official volunteer at the hospital/clinic and cleared to be there.
As far as volunteer opportunities, I would suggest doing something that truly interests you. It is good to find something "clinical" or medical field related to gain exposure and experience, but just as important is volunteering in something non-medical. Find something you are passionate about and start there. It is important to show your heart for service, as this is what truly makes a great PA.
Hope that helps!