My flight was delayed and arrived at BKK around midnight. Many of the ATM's at the airport had signs that they are closed from 11pm to 5am (or maybe it was 6am?)

That said, I still found a working ATM. I took out 10,000 which mostly lasted me the whole trip. I did take out 2,000 from an ATM in my hotel lobby the day before i left and had just a few hundred left at the end.

I use a credit union, which gives some of the best exchange rates out there, and reimburses any bank fees.