Where to get luxury ball in Pokemon LeafGreen?

You can get the luxury ball in Leaf Green by going to Lost Cave (a Surfing distance away from Five Island). You must locate Lady Selphy. She will battle you with two lv. 49 Persian. After defeating her, she'll take you to her house, which was the empty house you passed by earlier. Go talk to her when you have the Pokemon she wants to see, and she'll give you a Luxury Ball.