"Re-entry permit" & "fine for overstay" counter (office) is same one small office at Don Mueang airport. It is located after passport control and before security screening.
Tell the passport control officer when you are stamping out that you need a re-entry permit.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.