It looks like it may be restricted here (in Thailand) so you'll need to visit a hospital, doctor or specialized travel clinic.

I've seen prices quoted in the 150 baht/dose range; it might be called Malanil (GSK) here?

Most local health experts say that unless you plan to be in heavily forested areas after dark (where infection rates are high and transmission a possibility) that a malarial prophylaxis is both unnecessary and the side effects worse.

http://www.thaitravelclinic.com/Fron...-malarone.html

There are a lot of other, worse (OK, maybe not worse, but more easily contracted and potentially life-threatening) health problems here, Dengue and leptospirosis for example.