When we were there in October, Main Street bakery in Magic Kingdom had a few juice choices. I remember apple, grape, orange, grapefruit, V-8, and a few "blends". Animal Kingdom had a few choices also, can't remember the specifics. We didn't buy juice at Epcot or MGM, but I'm sure they had a few different kinds as well.
We stayed in Saratoga Springs, and they had the same juices as the bakery. allearsnet.com may have the specifics.
Have a great trip!