Fresh, a restaurant at the Dolphin, offers veggie juices in three flavours at breakfast. One is carrot based, one is a green and one has beet juice in it. I don't remember the exact veggie combinations. These were all laid out in small glasses for the buffet breakfast, available as part of an AYCTE continental or full buffet. I didn't see any wheat grass options, but wheat grass may have been in the green juice.
I'm not familiar with any actual juice bar locations in WDW.
Perhaps 'on demand' juices are available at Fresh at lunch or dinner, or at another location in the Dolphin?