For the random mystery islands with the nook tickets, I believe you only have a rare chance of finding one other type of fruit (I've heard this referred to as your 'sister native fruit'.). Otherwise, you will only typically find your native fruit on those islands. For the others, you'll have to trade. There's a lot of super friendly people here that would be more than willing to trade you some for free, so try asking in the airport boards! I'd trade you myself but I just started over yesterday so I'm still getting things up and running. if you still haven't found any after a few days though, please send me a PM and I'll gladly come drop some off for you