So usually when I try to buy adapters I end up buying one that doesn't fit or doesn't work for some reason. I need one for my casio keyboard because the 5 giant batteries it used died. I don't want to use batteries! The keyboard is a Casio MT30; here are some pics of the jack and the back of the keyboard. I wrote in red on the jack picture because it's hard to read.
I have the radioshack 9v adapter with the changeable tips and it worked fine with 7.5v casios i've had in the past. I think the plug is the same size as a boss pedal plug but reversed priority.