As a rule you go to the immigration office where the extension was issued to get a re-entry permit. Some offices will do it even if your extension wasn't issued there BUT some won't
You have to apply in person so if you're getting a re-entry permit for your wife she goes to
OR
You both just get re-entry permits when you are at Suvarnabhumi airport on the day you fly out. Re-entry permits are sold
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at the re-entry counter in International Departure 2