Buzzing noise tied to brightness

I bought one of these used and everything was great. But, after a few days of leaving it plugged in and off, I noticed a crackling sound coming from it. I unplugged it and the noise immediately stopped. When I turned on the computer/display a bit later, the noise was terribly loud and gets louder/softer if I raise/lower the display brightness. Some online research seems to indicate that it has to do with the power supply (I guess this is what happens if there’s no real “off” button). How can I go about replacing the power supply?