Where should a Canadian citizen residing in Samut Prakan apply for a Non-O visa for retirement?

You should go to that office and get the hand out they have listing the requirements to apply for the in country 90 day Non-O at at that specific branch (y)

You would apply for that 90 day Non-O visa at the immigration office that serves the area where you live, SO if you're in Samut Prakan (which you would know by your address/zip code) you'd apply there.

You need a minimum of 15 days left on your current stamp to apply there.

As you are a canuck you can get the Canadian embassy here to issue an affidavit of income from abroad notary letter stating you get 65K baht a month from your country and use that for proof of income rather than banking the 800K baht in a thai bank account in your name only