In every shopping mall there is usually a "banking" floor or area where you will find every bank and financial services company.
The top 5 banks here are:
Bangkok Bank
Krungthai Bank
Siam Commercial Bank
Kasikorn Bank
Bank of Ayudhya (Krungsri)
There are bank branches and ATMs everywhere.
I do not know which (any, all, some?) of these banks offer the interbank exchange rate at either their ATMs or their counter service - I just haven't had to stay current on this subject? Maybe review the relevant threads on the ThaiVisa banking forum to get the latest, best news.
I do get the VISA daily interbank rate (+/- a day or so depending) at AEON ATMs. They have a branch in the basement level of Terminal 21 at Asoke BTS, among other locations. (Amazingly, I get a notification from Fidelity on my phone before the cash is dispensed.)
http://www.aeon.co.th/aeon/services-...work-location/