I'm actually unsure. Maybe you gotta complete the farm? (i.e. getting Cindy to be pregnant)
What I do know is that you can find another to the east of the Desert Temple (I'd try solving the 4 riddles inside the temple, and the 5th by the brazier to find it.)
The two should suffice as it is 1 rare fruit to 1 Super Lactaid. That is until you can get Cindy to sell you some.
There is also a third in the ruins next to the entrance to the Dryad Temple, so I'd just say look around for a bit and you'll find some. (Just in case ya need some.)