churchmouse, with no family or friends around the incapacitated person becomes a "ward of the state". They are then placed in a nursing home or group home. It can takes months or years to find a relative who cannot identify their own family.
Going there must surely be the most straightforward way, yes; but there must also be a system for incapacitated people who don't have next of kin available? What is it?
I live in Texas. My elder law attorney told me that as long as a lawyer believes that a person understands and consents, then it is possible to execute legal documents. She told me about a man who had had a stroke, and was able to put an X on a will in front of witnesses. He died shortly afterwards and his will was valid for probate.
KFreeman, I presume by your question that he has never had durable and healthcare POA paperwork done. You do not say if you want guardianship or if you want to assign someone he and you trust in CA to be his guardian. You said your dad cannot sing documents or talk, but is he able to understand and communicate? If yes, a lawyer might still be able to have him legally make that decision, which would be hugely easier. If not, know that guardianship is a legal effort, and you could most easily start by finding an elder-care attorney in your area. Many will offer a free consult. He/she could then direct you on how to reach your goal to get your dad the care he deserves and needs.