I have heard that the Kona farmers' market has some more unusual things from time to time but it hasn't the few times I have visited it. There are a lot of things that exist here but do not seem to turn up at the farmers market. Most of what turns up is not terribly exotic compared to most of what appears in this forum. I made a list (by common names) of things I can recall seeing at the Hilo farmers' market (I am sure I may have missed an item or two, maybe someone will chime in):
abiu
atemoya (uncommon)
avocado (several varieties)
bananas (several varieties)
betel nut
black sapote
breadfruit
breadnut (uncommon)
cacao
cherimoya
chico
chupa chupa (rarely)
citrus (orange, mandarin, lime, lemon, kumquat, calamansi, grapefruit, pommelo)
coconut
dragon fruit (generally the yellow ones & the white fleshed ones)
durian (zibethinus)
egg fruit
green sapote (rarely)
inga (uncommon)
jaboticaba
jackfruit
lansones (uncommon)
litchee
longan
mamey sapote
mango (M. indica)
mangosteen
mountain apple
papaya (several varieties)
passionfruit (usually the yellow lilikoi but also sometimes some others)
persimmon
pineapple (yellow & white fleshed)
pulasan (uncommon)
rambutan (usually red but sometimes yellow as well)
rollinia
santol (uncommon)
soursop
star apple (green & purple)
starfruit
sweetsop (uncommon)
Obviously what you find will depend upon the season. I do not frequent the Makuu market down in Puna near Hawaiian Paradise Park but the times I have been there, they tend to have some more unusual fruits. I think that market is only on Sat (it is really more of a flea market). Hilo farmers market is open every day but Wed & Sat are the big days, with more vendors & better selection (also bigger crowds).
John