There are three conventions in circulation, and matching the right one to your destination is the whole game. The 1949 Geneva Convention IDP is the most widely accepted worldwide and the default most travelers need. The 1968 Vienna Convention IDP is required by a different set of countries, notably much of Europe and various others that ratified the later treaty, and it has a longer validity, up to three years but never beyond your underlying license. The 1926 Paris Convention version survives only for a tiny handful of territories.
Where to get one is refreshingly simple: only the authorized motoring organizations in the country that issued your license can issue your IDP, typically the national automobile association, and sometimes government transport offices.
Avoid every website selling international driver's documents outside those official channels; they're worthless at best and confiscated-at-checkpoint scams at worst.