At your local music store. If not in stock, they should be able to order or make you one. Or you could learn how to use a soldering iron (which is a really good idea if you see music & recording as having any continuing part in your life).
if your local store carries Hosa products, they either carry it or will order it expediantly for you. I needed a 1/4"stereo male to two 1/4" mono l/r females and it was under $5 to order it.