hi Isling it's simple answer, just try one of the apps from http://www.get4x.com/ website, there is android and iphone version for these, i recieved notification they have now more than 50 000 local listings
It depends where you actually are. Yesterday i changed money in close Nana Station. But when i was at Asok station (see bts map) i found a good rates in Terminal 21 - shopping center. I am located close 51 Sukhumvit Soi 26 in one hostel and there was one exchange place close to the bank too.