Unless I'm mistaken Nakhon Phanom is one of those offices that don't issue in country Non-O visas (not for marriage or retirement) ๐
So you do indeed need to go to a thai consulate in another country and apply
You need to show 800K baht in a thai bank account in your name only to apply for the 90 day Non-O visa at the thai consulate in Savannakhet Lao (which is across the river from the thai province Mukdahan ๐ )
These are the requirements from the thai consulate in Savannakhet, (update your thai bank book the day before you cross into Lao so it has a fairly current balance)
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