Thai people are very friendly and very well informed regarding available rentals in their area. It is always a good idea to ask the locals for some recommendations.
Newspapers
Property classifieds sections can be found in local newspapers like Chiangmai Mail or Bangkok Post (in English).You can also check the Housing Section at Just Landed (housing.justlanded.com/en/Thailand for Thailand).
Real estate agencies in Thailand
Estate agents in Thailand tend to dedicate their time to rentals of 30,000 Baht and more. Take a look at the website of The Real Estate Association to find out more.
If you are renting or buying property through a real estate agency, you will probably be required to provide a “character” reference (e.g. from your employer or some friends) and a bank reference. You may also be subject to a credit check. If you are buying a property, the commission for the estate agent should be paid by the seller and is approximately 3% to 6% of the selling price. Many estate agencies list the same properties, so make sure you know which properties you have visited to avoid repetition.
Internet
The internet is of limited use when researching properties because not all properties are listed online. Properties that are listed are either very expensive or the sites they are listed on are so badly put together that it is very difficult to understand them. However, sites such as ThaiApartment have rentals listed in English.