re-entry permits are sold at Suvarnabhumi airport in Departure 2, tell the people when you go thru security you need a re-entry permit and they will get someone to staff the desk (it's open
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but sometimes no one is there)
The price for a re-entry permit is 1200baht for a single re-entry and 4000 for a multiple re-entry. Keep in mind a re-entry permit will get stamped back in until your current expiration date when you come back.
Look for that sign