After three days of mind-blowingly farming the Mystical Source in Whimsyshire, I have finally looted the oh-so-very-elusive Mystical Source.
I don't have the Sage's Journey Set recipe. I simply wanted to loot it to demonstrate whether it existed in closed beta or not. Or who it could be looted from. Of course, also to add it to the Diablo III: Reaper of Souls Legendary Crafting Materials guide.
It drops from a super-rare pony, and I don't know if he spawns in normal mode. After nigh 80 runs in normal mode, I switched to Torment I mode. After 20-30 runs this pony spawned and dropped the Mystical Source. Check out the images at the Legendary Crafting Materials guide. The link is accessible on the right menu under Diablo III: Reaper of Souls --> Legendary Crafting Materials (for future reference).