What's the difference between the different versions? Read my article, A Brief Survey of Thai Bible Translations.
Living Outside of Thailand?
If you are living outside of Thailand, and want to buy the 2011 Thai Standard Version, you can order directly from the Thailand Bible Society in Bangkok. They will ship internationally to you. The postage will be expensive but you'll have lots of choices in Bibles (Thai only, Thai-English, NT only, etc.). The Thailand Bible Society produces the Thai Standard Version that is used in the majority of Thai church. To order, click here to go to the Thailand Bible Society website.
The New Thai Translation Version (NTV) is available from Kanok Bannasan (OMF Publishers Thailand), and can also be shipped internationally. Please contact Kanok to confirm availability and pricing. As of 2016, the NTV includes Genesis, Pslams, Proverbs, and the entire New Testament.
Alternatively, you can try ordering from a Bible society or website (such as Amazon.com) located in your home country. I've included below links to some places in the USA and UK where hard copies of Thai Bibles can be bought, as well as links to digital versions.
In the USA
Thai Standard Version is available from the American Bible Society.
In the UK
In the UK, you can get either a full Bible or a Thai/English New Testament from the British and Foreign Bible Society.
Multiple versions of the Thai Bible are available online at the YouVersion website.
The Thailand Bible Society has a Thai Bible iPhone app in the iTunes store.
The Thai New Contemporary Version (similar to the NIV) can be read online at the Biblica website.
The Thai King James Version (KJV) is available for purchase or download from Philip Pope's website.
An audio Thai Bible, a free MP3 download is available at Faith Comes by Hearing