What an IDP is, why you need one to drive in Thailand, and how to get a genuine one before you travel.
An IDP can only be issued by an authorised body in the country that issued your driving licence, and you must obtain it before you travel. It cannot be issued once you are already in Thailand, and no service can legitimately sell you one online while you are abroad. Treat any site offering to do so with suspicion.
An International Driving Permit is an official translation of your existing driving licence into several languages. It is not a licence in its own right and carries no driving entitlement on its own. It is valid only alongside the original licence it translates, and both must be carried together whenever you drive.
Apply through the authorised issuing body in the country that issued your licence. Most issue a 1949 or 1968 Convention permit; check which one the issuer provides and confirm its current requirements with them directly, as these change.
Not from one of these countries? Contact the automobile association or licensing authority in the country that issued your licence.