I used to have a good supply of old plugs and sockets.  I would buy
old extension cords at garage sales.  As time went on the demand was
greater then my supply.

Here is what you can do.  If your piano has a normal two prong male
socket attached to it and the new female ends are too big in diameter,
the diameter of the new female can be turned down to fit in the socket.

In this particular case you will probably find that the there is a
socket inside your piano where your electric motor plugs into the BX
cable which is attached to the inside of the case.  This socket can be
used to make your extension cord as it will most likely match up with
the odd male you cannot find a mate for.  It's easy to put in a new
socket inside the piano as most electric motors use standard male ends.

Paul Manganaro