Thailand has 3 main budget airlines which fly domestic routes.
to AirAsia.com...now everyone can fly... to NokAir.com -Low fares airline Orient-Thai Airlines
I have used Air Asia flight a lot and Nok Air once, no problems booking online in my experience but Air Asia once lost one of my bags on a direct flight from CM and also once landed in completely the wrong part of the (old) airport on a flight from Penang. I think the pilot forgot which airline he was flying, because they kept saying thank you for flying Thai Air. They don't have a very good reputation for being on time but the last flight I took with Air Asia a month or so ago was spot on leaving and arrived in advance. Air Asia has leather seats and quite new planes, can't remember what the Nok Air plane was like...pretty OK I think though the flight was delayed.
Although Orient Thai (One-2-Go) has been in operation for several years they have a
really **** user-unfriendly website.
I've never yet managed to book a flight on it. Too bad for them if they lose zillions of customers because they can't can't get their act together!!!!!!!!!!
You are best booking Air Asia and Nok Air online as much in advance as possible to get the best deals rather than waiting till you're in Thailand. You can get tickets at the airline offices at the airport too though.
Other airlines which fly domestic routes are:
Thai Airways Bangkok Airways :: Asia Air Travel, Airlines, Flights, Samui Thailand
If you require free food and drink during your one hour flight, you will get some on these two but your ticket will cost twice as much. Bangkok Airways has a monopoly on the Airport in Samui but you can fly to Surat Thani on Air Asia and get the ferry from there if you want to cut costs.
You can also get overnight sleeper trains to Surat Thani and Chiang Mai. Tickets can be obtained either from travel agents in BKK or from Hualumphong train station. Travel agents charge a small commission of course, but save you having to hop on the MRT (underground) and go and pick up the tickets yourself. Sleepers need to be booked a few days in advance, especially during peak season and, if you're going to Surat Thani when there's a wretched Full Moon Party on Ko Phangan.
I have only been on one train that actually derailed but they are often late arriving at destination. The sheets etc are clean, they serve you meals and beer but the toilets and washing facilities aren't Hilton standard.
There are several types of buses. The most comfortable ones are 1st class VIP buses (24 seats) but these are mainly overnight buses. They have toilet, videos and airline type hostesses who bring you drinks, snacks and sometimes an entire meal (included in the bus ticket price). VIP buses with more seats do too...what you get depends on which company you choose. Then you get ordinary A/C down to rickety fan, lots of different types of buses in Thailand. However I would strongly advise you to get tickets from a government bus station rather than a travel agent. Any bus that goes anywhere near Khao San Road should be avoided like the plague unless you are willing to risk having your bags in the hold relieved of a few items while you sleep. Minibuses that go anywhere near KSR should also be avoided, cramped and they tend to have long rest stops at their buddies eateries en route.