A Yamaha FC4 or FC5 should work fine on your PSR 2100 mike. They are very good pedals from what I understand. For a Volume Pedal try the Yamaha FC7. PS: Both the FC4 and FC5 can be used as an Expression pedal too.

If you can't find a store that carries the Yamaha pedals, have your local shop guy order you them if he can.

Best regards,
Mike