Can I get my passport notarized in the US for use in Thailand, or must it be done at the US Embassy in Bangkok?

First, did you get the list of required documents from your Amphur?

Some like Mae Rim don’t require it.

Second, do the requirements specifically ask for your passport to be authenticated by the Thai embassy/consulate?

Either the embassy in Bangkok or the Consulate in Chiang Mai. It’s a 3 minute walk in $50 USD service.

The real hassle is getting the certified copy translated and then authenticated by the Thai Ministry of Foreign Affairs.

Pro Tip: make sure you are happy with the translation of your name into Thai because it’s very difficult to correct the translation later after it’s on 50 other documents and ID’s.

As for as I know, a passport is a legal notarized document. Only copies have to be authorized.

For the same purpose (I did not know it was needed) I had mine authenticated in Australian Embassy in Bangkok, 800 THB, on the spot, took minutes, no appointment.