The usual charge for a notary is 2000 baht but police and doctors are usually cheaper.
I have signed proof of life forms for UK pensions but DWP have eased up on people in Thailand they realize it can be difficult to get Thai people to sign, especially if you live out in the sticks.
Who can certify a document
Your document must be certified by a professional person or someone well-respected in your community (‘of good standing’).
You could ask the following if they offer this service:
bank or building society official
councillor
minister of religion
dentist
chartered accountant
solicitor or notary
teacher or lecturer
The person you ask should not be:
related to you
living at the same address
in a relationship with you.