I've heard that a native delicious fruit sells for 600 bells. As for how to get them, I think there's just a random chance that any of your native fruit trees has a chance to grow one, and you can then plant it to grow a delicious fruit tree that only grows delicious fruits.
I've heard a theory that every town starts with one delicious fruit hanging from a tree. Either that or you just start with so many trees in your town that the odds of finding one are high.
http://i45.tinypic.com/fpchec.gif http://img149.imageshack.us/img149/9413/cutepuppy16.png - Number of people that D'awwed: 70