Exchanging money on Thailand's islands
Koh Samui, Koh Phangan, Koh Tao and Koh Chang: no booth there publishes its rates, so here is the reference rate and where to change instead.
Thailand's islands are where the published rate runs out. Almost every booth on Koh Phangan, Koh Tao and Koh Chang is a single family business with a painted board and no website, so there is nothing to read automatically and nothing to rank honestly. Rather than invent numbers, these pages give you the reference rate for the day, the best published rate on the nearest mainland, and how much cash to arrive with.
Island rates are worse for reasons that have nothing to do with the shops being greedy: a booth with no competitor fifty metres away, banknotes that arrive by ferry, and customers who cannot easily walk out. Expect to lose a few percent against a city rate. Koh Samui is the exception, because the Government Savings Bank counter at the airport publishes a board, and that one is tracked.