The fifteen days visa, that at the time of writing is possible to get at any of the land borders with Thailand, is definitely too short.
Getting into the country by airplane guarantees you a 30 days visa, but if you really want to enjoy the “Land of a Thousand Smiles” and you are travelling around Southeast Asia it is worth spending a couple of days in one of the capitals of the neighbouring countries to pick up a much more suitable 2 months visa.
Here is a list to all the embassies in the Southeast Asia area, and a few links with some up-to-date information related to best practice to get a 60 days visa for Thailand.
One of the best sources of information for Thai visa issues and questions (or anything related to Thailand) is the busy forum of the web site Thai Visa . It is definitely worth a visit.