This cave is found later on in the game, I believe it takes place sometime after you beat the 5th gym leader. Should have made that clearer the first time so sorry about that.
So I have been trying to evolve my pichu for the water gym leader and it is currently lvl 44 but does not seem to have enough happiness soooooooooo what would be the easiest way to raise its happiness now that its almost at its level cap