Sorry, I have no idea where to find such parts. You can try searching around for Spare Rebel Parts, but most of what I see are more fundamental pieces. But unless you're willing to take it apart and put it in yourself you're probably going to pay someone more than the camera is worth to fix it. You could try to find a camera repair place, like an old school guy-in-a-room-full-of-old-camera-parts type of place, where he might be able to shove something in there for a reasonable price. If you're mechanically inclined you could take it apart, pull a button off something else, and stick it in there. But I don't know if there are any other buttons of less importance on there, most seem more used to me.
Do you shoot in RAW? My recommendation would be to set the WB to auto and leave it there, and shoot in RAW so you can adjust in post anyway. You should be able to adjust the WB through the menu using the navagation buttons.
