What is the best route to find a physician assistant that will allow me to shadow them?

Contact the nearest clinic or hospital that has a PA on staff and ask them if you can shadow them

Go to your local military healthcare facility. If you are not near one, you will have to shoot for the VA or a Private facility willing to allow you to shadow. This would not be easy considering confidentiality, if you did not work at the facility I would not allow you to sit through mine or my families care sessions. Good luck. Thank you for your service.

Go to Womack and talk with the one of the hospital administrator or even your medical PL or Platoon Sergeant if you have one. You could also contact your local AMEDD Recruiter if that is the route you are trying to go and they might be able to also put you in contact with someone.