Bangkok answers
How do I get a Thai SIM card?
Quick answer
Bring your passport to any operator shop or airport counter. Tourist packages are sold on the spot; longer term plans are easier once you have a local address.
Thailand requires identification to register a SIM, so take your passport. Airport counters are convenient on arrival and sell short tourist data packages, while operator shops in any mall will set up a longer term plan and can port a number later.
If you plan to stay, it is worth switching to a monthly plan once you have an address, since data allowances are larger and top ups are automatic. Coverage across central Bangkok is strong on all major networks, so choose on price and shop convenience.