Where can I get my marriage certificate notarized in Bangkok for my non-O visa extension?

I am not an expert, but you will need it translating first and then notarised at the Department of Consular affairs.

If you go there are people that will do the translation in an hour or 2, but phone and book an appointment for the afternoon at the Consulate so that you can do it all on the same day.

If it was me i would take a visit to The Trendy Building on Soi 15 I think Sukhumvit. All kinds of services available there.

We had to do this to register our marriage at our local amphur in CM.. since we were married outside of Thailand.. we engaged an agent to do everything.. gave them our marriage certificate and copies of our passports and my wife's national ID card.. they handled everything from start to finish.. the original marriage certificate has to be certified by your own consulate / embassy first.. and will be translated into Thai with the Thai version then being certified by the Ministry of Foreign Affairs in Thailand.. you will then receive both of these documents as well as your original.. and in our case the agent included a translated copy of my passport (into Thai) in case of future need.. and we paid around 5,000 bht for everything.. it's really not worth trying to diy since its too time consuming and would take too long.. we had it all back inside 2-weeks!

It needs to be translated to Thai by official translator and then you can also hire them to send it to the Ministry of Foreign Affairs to be certified. Otherwise doing it yourself can take longer. There are several translation offices in Bangkok, especially near the embassies.

Attorney office?