89 year old uncle lives in the Mission. He has some kind of growth on his scalp (skin cancer?). Where can he go where Medicare is accepted?

Medicare and Dermatology. Medicare Part B (medical insurance) generally covers doctor services when medically necessary to evaluate, diagnose, or treat a medical condition. So if you're going to a dermatologistto treat a medical skin condition, your visit might be covered.Sep 3, 2017

There must be a tool online to find a doc that takes Medicare.

U.C.S.F. ???

UCSF (888) 689-UCSF

Even the best hospitals take Medicare.
If he qualifies and requires surgery, he could apply for Medi-Cal pending.

I'm surprised you are having such a hard time finding a doctor that accepts medicare in SF. Worst case, go to the ER. They have to accept medicare. It's the most expensive way to treat it but if you have no other way, it's better that your uncle gets treatment at high cost than no treatment at all. Unfortunately there will be a copay with medicare. If he doesn't have supplemental insurance then it can be quite substantial.

Doesn't SF still have universal heathcare? It had it long before Obamacare. Any SF resident gets affordable healthcare depending on their ability to pay. If that ability is $0 then it's free healthcare.

https://healthysanfrancisco.org/