As with most low-cost airlines, always shop only on the airline's website. The best bargains are available 1 to 2 months before departure, but in low season you can get decent prices a few days before your trip.
The cheapest one-way domestic tickets within Thailand start at Thai Lion Air at around ba 26 eur, the more expensive ones at around a thousand. There are always several flights a day between major destinations (Bangkok - Phuket for example).