I have just bought 50 Doxycycline tablets for £39 from the 'well travelled clinic in Liverpool' they do have a web site - www.shop.welltravelledclinics.co.uk although we did have to see someone first. She showed me a great web site where you can view a 'malaria map'. www.fitfortravel.nhs.uk - go to 'destinations' - click on the map at the 'east asia region' then click the country you want then select 'malaria map' - you will see that Thailand is low to none risk and anti malarias are not usually advised. remember tho that you need to take for high risk such as Laos, Vietnam ect 2 days before and 4 weeks after so maybe try and plan your trip to visit high risk one after the other. this is the advice I have been given by the well travelled clinic which is a subsidiary of the Liverpool School of Tropical Medicine. See you in August - cant wait!!
Most of the long-time travellers to the region I know don't take anti-malarials. It is a very personal decision, so I'd never say take them or don't take them, but I'm comfortable saying that I don't take them. The problem for me is that I'm not convinced that the potential health benefits are worth the potential health risks or the financial costs.
Instead, what I try to do is use a good bug spray, cover up when needed - particularly at dawn and dusk - and sleep under a mosquito net. I know others have also commented on how important it is to start out healthy and maintain a healthy diet rich in fruits and veggies, which is easy to do on the road in southeast Asia.
In my experience, other insect-borne diseases, like Japanese Encephalitis, are a bigger worry than malaria, and while the anti-malarials can't help with the other stuff, covering up and using bug spray can.
Again, personal choice. Do what makes you comfortable and best fits your style. Cheers.