Confused here, as many have said any branch can give you the letter. Or do you need the original documentation about from where your funds arrived in Thailand from? Mine come to my Bangkok Bank Branch, and if your account is in Bangkok there maybe some truth to this. I have found the Jomtien branch of Bangkok Bank to be quite helpful at all matters
Living and traveling for a non-vaccinated individual can be very difficult if not impossible in some parts of SE Asia, don't expect this to change soon