Any music store should sell them. In my store I buy Hosa brand and Signal Flex brand models. Some models are one piece units with the correct ends on both sides and other companys make them where instead of a 1/4" male plug coming out of one end, they put a 6" cable attached with the 1/4" plug at the end of the cable. This system is more popular for some because the transformer itself hangs down from the keyboard as opposed to the transformer sticking far out and away from the keyboard. I would also think most Radio Shacks would sell them as well. It called a low to high impedence transformer.
George Kaye
Kaye's Music Scene
Reseda, California