You have the link to the one you bought on Amazon? Just out of curiosity. AC/AC adapters are just a little less common.

Any normal 9V AC adapter intended for the US market will work, so long as it has the right sized barrel jack and provides at least 2 amps. More amps isn't going to hurt anything. I'm guessing the piano is drawing too much current(amps) for that weird dc inverter board.

This one looks fine and has the transformer in a separate housing over a wall wart, so long as the barrel is the right size.