You can get them by sending villagers a short letter with a present attached. You don't always get them though

I did some research on this a while ago. Sending any villlager a letter with the message "Please send fruit." will result in getting either clothes or fruit, including coconuts and foreign fruits. I'm not sure how it's determined what will be sent back, but I've found that you seem to get more fruit the longer you wait to send out more letters. It's also required that the letter not be "gibberish," so any shortening of that phrase that I've tried resulted in getting nothing but a complaint letter back.

The strategy I always used was to send a letter with the words "friend friend friend friend friend" and attach a fruit. Used to work pretty well