I don't know what the USDA guidelines for shipping cleaned seeds back are, but the Kapiolani Community College Farmer's Market on Saturday mornings and Frankie's nursery are both a sure bet for finding a diverse variety of locally grown tropical fruit.
The KCC farmer's market is the largest on the island. Frankie's has a stand, and I've seen everything from wax jambu to marang on it. But, Frankie's sells a TON of rare fruit at their nursery that you won't find at the market. I was there two years ago around this time, and they had Fukuda, Keitt and Rapoza mangos, bananas, mangosteen, rheedia, canistel, mafai, longan, and that was probably half of the selection.
I would also check out the other farmers' fruit at the market. A lot of it will be lychee and mango and you won't find grafted trees, but if you want fruit and seeds, those are the two places I'd go.