Where can I buy the SIM cards? Is it better to buy at the airport or in the city?
You will see the telco sales counters in the arrivals area of Suvarnabhoomi airport (once you exit the the customs area). DTAC and AIS are next to each other. If you walk another 200 meters or so you will see the less busy TRUE. You can also buy the SIM cards from the 7/11 counter in the basement (same level as the train station to the city). At the Don Mueang and other international airports, check the arrivals area or any convenience store like the 7/11s.
You can also buy SIM cards from the mobile shops in the city. I would recommend buying the SIM at the airport, as the counter staff will be able to set up your phone and advise you on top ups. The convenience stores in the city tend to be busy and the staff will not configure your phone. The phone shops are all right, but the staff may not know enough English. Having said that, the SIM cards do not need any configuring. I have found that the internet access works as soon as I slot the SIM card on my iPhone. On my Android phone, I had to reboot the phone to get the data connection to work.
What are the data enabled SIM card options for Thailand? Following are the data SIM cards you can purchase
DTAC’s happy SIM card. More details at https://www.dtac.co.th/en/prepaid/
AIS 12 call is another option. Again, this SIM card is available at shops all over the city. See http://www.ais.co.th/roaming/visiting/sim-for-traveller-en.aspx
True is the third option. http://truemoveh.truecorp.co.th/3g/sim-truemoveh/prepaid/entry/2258?ln=en
True's SIM card for tourists.
Most of these SIM cards will allow top-ups that will increase the validity beyond a week. Just let the sales person at the counter know how long you are going to spend in Thailand, and they will recommend a suitable plan to you. You can request them to show you the recharge steps.
I have used SIM cards from all the networks. Coverage on all these networks were good ,though there were some areas with slow or no connectivity - for example the outskirts of cities, on highways etc.
Do I need to show the passport or any other papers to get the SIM card?
Some places ask for you passport, some don't. Better be prepared with some sort of ID.
Will the SIM work for a SIM locked phone or a CDMA phone?
No, your phone has to be SIM unlocked. The SIM cards will only work on unlocked phones from Japan, South Korea and some US networks. If you are unsure, please call your network provider before you leave your home country. They might have an inexpensive data roaming plan that you can use for short trips. The networks run on GSM technology so the SIM cards will not work with CDMA phones.
Will they have nano SIM card for iPhone 5s, micro sim card for iPhone 4s and others?
Of course, they have. If not, they will cut the SIM card for you.
DTAC's SIM card in regular and nano size.
Thai mobile users are ahead of the curve. They are heavy users of latest smartphones and the phone shop staff are familiar with latest phone models. Back in 2007, Thai schoolgirls were one of the first demographic in Asia to use the iPhone. Suvarnabhoomi airport is one of the most instagrammed building in the world.
What is the best place to buy a new, unlocked phone in Bangkok?
Try MBK Centre http://en.wikipedia.org/wiki/MBK_Center or Phantip Plaza http://en.wikipedia.org/wiki/Pantip_Plaza. Thai mobile users are always experimenting with new phones. So you can always find newer phones in the second hand market too.
Is it easy to get free wifi at cafe and restaurants in Thailand?
You hotel or hostel will surely have wi-fi. The more upmarket the hotel, the more stingy they are with wifi. The number of cafe and food places offering free wi-fi has gone down in recent years in Bangkok. Either they don't offer wifi, or the network does not work. Chiang Mai and smaller cities are much better with respect to free wifi in cafes.
If you have questions, email me at email@example.com.