Should I get a re-entry permit at the airport or an immigration office in Bangkok?

You can obtain a re-entry permit either at Suvarnabhumi Airport or at an immigration office in Bangkok. If you decide to go to the airport, the re-entry counter is located in International Departure #2, after security but before passport control. It generally requires just your passport, and costs 1200 baht there (including a 200 baht service fee), while it is only 1000 baht at the immigration office. If you are leaving from Suvarnabhumi directly, getting it at the airport is convenient. However, if you're transiting through Bangkok from another airport, it might be better to obtain it in advance at the immigration office.