Can you purchase a re-entry stamp for a retirement visa from any immigration office in Thailand?

Yes, you can purchase a re-entry permit for a retirement visa from any immigration office in Thailand or even at some international airports prior to your departure. The cost for a single re-entry permit is 1,000 THB, and for a multiple re-entry permit, it is 3,800 THB. However, it's advisable to check with the specific immigration office you plan to visit, as some may not issue re-entry permits for long-term extensions granted by offices in other provinces.