I don't know of a specific doctor, but you may have to go concierge. A difficulty, though, is she at the borderline for a pediatrician and a GP that serves adults. I would try for the latter. Also haven't heard of a concierge pediatrician practice.
From experience, be prepared to have doctors doubt it is a medical problem and suggest it is psychiatric. I think the right specialist is key. Often these nonspecific come from an autoimmune disease, but too often GPs just run tests for ANA and rheumatoid factor and if those tests are negative they conclude it is not autoimmune. But there are plenty of autoimmune diseases for which those tests are invariably negative.
Perhaps you could give her symptoms and we could give suggestions as to possible specialists or at least a starting point with the GP.