Can I purchase a re-entry permit at any immigration office in Surat Thani?

You can usually apply for a re-entry permit at any immigration office where you did your 90-day reporting, where you received your extension, or where your TM30 is filed. In Surat Thani Province, there are three immigration offices: the main one in Surat Thani, one in Koh Samui, and one on Koh Phangan (though it's uncertain if the latter sells re-entry permits). Additionally, if your visa is a long-term one (like a Non-Immigrant), it's advisable to get the re-entry permit from the immigration office rather than at the airport, although you can do so if departing directly from Bangkok.