Try the 7-11 for the Sim card. I got one there last time and it worked fine. Lots of WiFi around too but can be slow in some places. If your phone has GPS it will work on the maps if you get turned around.
Get A Thai SIM Card At Bangkok Airport
The easiest solution is to do it as soon as you arrive at Bangkok’s Suvarnabhumi airport. There are branches of all three telecoms companies – AIS, DTAC and True – in Bangkok Airport’s Arrivals Hall so you can get a SIM as soon as you arrive. Ask them to install and set up the SIM card for you, just in case any settings need to be changed. Ensure the data connection is working and you can access your emails, Facebook etc before you leave the shop. These shops stay open late so check them out even if you arrive in the small hours of the morning.
Getting A Thai SIM Card In Bangkok
If you don’t get your Thai SIM at the airport, you can find branches of all three mobile carriers in Siam Paragon, Bangkok’s ritziest mall where the staff speak English. The mall is located right next to Siam Square BTS Station – very easy to find.