You might find the following resources useful:
This practical dictionary is designed to help English speakers communicate in Thai. It is equally useful for those who can read the Thai alphabet and those who can't. The transliteration system is the same as that used in Paiboon Publishing's other books.
Many more dictionaries can be found at Thai Hypermarket.
An alternative to buying paperback dictionaries, you can choose between a variety of electronic versions that are small enough to carry around in your pocket! Click here for more information.