Any EP Cardiologist (electrophysiologist) at a big teaching hospital should be able to handle POTS these days. If you're not satisfied with the first doctor you see, try a different doctor. It can sometimes take until the third or fourth doctor you try before you find one you click with. Once you get on a good steady plan of care from a specialist, typically you will only see them once every six months to a year for follow up and your PCP will take over following your case for ongoing treatment.
If it is a particularly complex case and full autonomic testing is needed, you will have to travel at least as far as Mayo Arizona. They have many but not all autonomic testing available there, and it is the closest autonomic clinic to your location. There are none in California, unfortunately. If you want all of the tests available you'll have to travel as far as Cleveland, OH for Cleveland Clinic or to the Mayo in MN. However, it is unlikely that she will require this level of testing so make sure you exhaust your local resources first.
I know this thread was awhile ago but I believe Stanford in Palo Alto does tilt table testing for POTS. They have an Autonomic Dysfunction Program. My son (14) was just diagnosed at Mayo Rochester. I would love to find some support with other parents. This is heartbreaking.