As KhunBenQ said, you can get the 30 day extension at immigration office. In my own personal experience it was not a difficult thing to do and at Khon Kaen immgration office. When i got my extension sorted I did it within the first week of entering Thailand. I then knew that the next 11 weeks were sorted and I didn't spend rest of trip wondering whether or not the extension would be granted. By the way, the cost was 1900 thb, payable and receipted at immigration office.