loveisneverkind

You can only get the melon from Lava Lava Island, but it's not something you can just find somewhere on the island (trust me I checked). Once you have beaten the boss in the volcano and have gotten the final bu-ulb seed, (To get this you have to get the treasure that shot out of the volcano and give it to Kolorado) you can finaly get your hands on the melon! All you need to do is have a food item made by Tayce T. (I don't think Tasty Tonic or Super Soda works because you can get those from other places), and talk to the (adult) Yellow Yoshi. When you do, she'll say something like "~sniff sniff~ what smells so good, can I have it?" - give her the item.

(I would suggest that you give her somthing not very usefull or something that's easilly/cheaply obtained - ex: Koopa Tea or Nutty Cake, because you can get Koopa Leaves and Goomnuts whenever you want for free) After you give it to her she will give you a melon (She won't ever give you anything else, no matter what you give her, so don't give her anything you really want).