There are numerous international and local car rental companies. Larger companies have offices in Chiang Mai, Phuket, Pattaya, Hua Hin and Hat Yai and other main towns and cities.
Rent a car isn't the most popular option for travel in Thailand, however is safer than renting a motorbike. You will need an international driver license. There are also a larger number of mini-vans available for those traveling in groups.
It is recommended to look other alternatives before renting a car; the roads in Thailand are not very safe for those unfamiliar. Although traveling by car definitely has its advantages, you can rent a car with a driver, this makes a good option for a day trip to out of the way place.