Been to a nephrologist, who else would specialize in hypokalemia?

Endocrinologist. Potassium homeostasis is primarily determined by the hormone aldosterone so an endocrinologist might be able help.
Hypokalemia. An endocrinologist may help with diagnosis.
Who to see for low K. Any good internist should be able to handle most K problems. Endocrinologist and Hypertensionologist. There are several HTN experts in LA. Go to Am Soc of Hypertension website and find one. I live Lake Tahoe on Nv side and can see you but I am in and out. Better to find someone there. But happy to consult there with you as this is all I do.