How do you get durable power of attorney over somebody who cannot communicate?

You don't.

I am assuming that by "cannot communicate" you mean that the person is not able to take in information, and equally is not able to express himself.

If that is so, he cannot possibly think through the decision as a consenting adult and give power of authority to someone of his choice.

In any case, you do not "get" DPOA. It must be intentionally and freely given by the person you would be acting for.
You do not have power of attorney of any sort "over" somebody. You exercise it *for* them, acting on their behalf, in their best interests, and as far as possible as you believe they would have acted for themselves.

If you need to make decisions for a person who cannot communicate, and you have to do it now and can't wait for that person to recover, there will be other legal ways to do it. You can apply for guardianship, but there are also temporary types of guardianship if this is only for the purposes of an emergency - speak to the person's medical or hospital team and ask where you can get advice on what to do.

You don't need an elder attorney to draw up a POA. Any attorney is able to do a POA.

The problem is if Dementia/Alz is involved. If so, the person is not capable of assigning anyone. Guardianship would be the next step and that is expensive.

Is this person able to write or just not talk?

I was wondering something similar, can a dementia patient give someone POA? She is in early stages and only has short term memory issues, but otherwise functions well.
Is POA good enough, or is guardianship somehow better? I know guardianship takes a long while via the courts?

See an elder law attorney. Very likely will have to be assigned as guardian or POA for health and medical. Is this a new situation, ie sudden dementia or stroke? Does the person understand, but is unable to speak? Is this person in a care center of some type and are there personnel there such a social workers to give you some guidance?