In most games in the series, Coconuts are the only exotic fruit available. Coconut-bearing trees can be found on Animal Island, and their fruit is taken back to the mainland to be planted by the town's beach.
Also, you can get any of 4 basic fruits, and then one sister fruit from a mystery island tour. Many players like buy ACNH Items. Coconuts can also be found in a mystery island tour, so that's already 3 of 5 found. While there is no set rare fruit, the ones that are the hardest/impossible to get are the two basic fruits that weren't either your original fruit or your sister fruit.
If you're looking to get these two fruits, I would suggest calling on a friend who has them, or buy Animal Crossing Bells to exchange them.