It's on Feelin' Alright: The Very Best of Traffic.
Sorry, Adam. I just noticed you wanted the live version. This is not the case with the compilation CD. I don't know if it is available on CD or not, but it was on a very old LP I used to own called Welcome to the Canteen. Good luck.
There is a live Mr. Fantasy on Welcome to the Canteen, although I'm not sure it's the one you're specifically referring to. Although there is a version of this CD out there now, I would wait until the middle of this month as it is apparently going to be rereleased at that point. Usually, on Traffic albums, the rerelease has additional tracks. Both can be ordered at Amazon.com.