The cave is in route 103 after you beat the elite 4. You can get these pokemon:
Zubat
Mareep
aipom
Pineco
Shuckle
Teddiursa
Houndour
Stantler
Smeargle
You can get the pokemon by using wonder spots. However you would have to use either mystery gift under nintendo events or a cheating device to activate the wonder spots.