You can download and print a letter from the U.S. Social Security website that shows your monthly SS income.  Will immigration accept that as proof of income for extension of permission to stay (original visa is Non-Imm O-A), or is something from the U.S. embassy also needed?

I have this, other than the letter from the US embassy declaring ur income,  ( costs $50) they have NEVER asked for anything else.

No never, Immigration demand your embassy´s confirmation on their own letterhead...

 

glegolo

You have to use the notarized letter from the American Embassy.

Immigration will only accept a income affidavit done at the US embassy or the consulate in Chiang Mai.

Info for affidavit is here. https://th.usembassy.gov/u-s-citizen-services/local-resources-of-u-s-citizens/notaries-public/income-affidavit/

You can complete it online and print it out. The fee is $50 or 1700 baht (as of today).

You will need to make an appointment for notarial services. 

You can only use the SS letter when applying for an O-A Non Immigrant Visa in the US

 

As others have stated,  here in Thailand you must use the Embassy Letter or money in a Thai Bank to prove income to Immigration