I don't know about actually getting hold of one, I got mine in Thailand, but once you have got one, you can set up international roaming via the AIS website https://12calleservice.ais.co.th/EPPWeb/index.html
Texts, and everything else, via international roaming is quite expensive though. So you might want to use one of the cheap calling methods, such as www.localphone.com or whichever one you prefer.
I can send texts to Thailand for I think it's 1.8p?
Direct from my UK mobile, it's about 20p and not much cheaper from my AIS SIM card.
Another thing is that if you don't use it much to call out, they can block outgoing calls, so it might not work for long.
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Oh, and you can top up via the same AIS site, but they charge you for using your card, and your UK bank also charge you for a foreign transaction, so it's quite expensive.