Hey there. There are 2 places I can tell you about for veggies and fruit. On South Kihei road just as you come into Kihei from Kahului there is a farmers market that operates roadside 3 or 4 days a week though I cant tell you which days. No fish there but nice selection of fruits and veggies and some home made baked goodies too. There is also the Maui swap meet that is on Saturdays in Kahului where there is a small area dedicated to fruits and veggies and there are some excellent exotic fruits there if your taste runs that way.
As for fish, I dont have a market recommendation but I often buy my fish at a place called Eskimo Candy in Kihei. They are the largest Fish wholesalers on the island and have a retail operation there. The fish and poke are great, as are their marinated kalbi short ribs.
I second the retail fish counter at Eskimo Candy. Their plate lunch isn't too shabby either.
For veggies, you're going to find the biggest selection on Saturday mornings at the swap meet in Kahului. It's moved from next to the Post Office to the Maui Community College campus, but there's still tons of great produce from all over the island.
If you eat red meat, find the small refrigerator case in Long's Drugs (Azeka Marketplace, on the mauka side of S. Kihei Road) that has Maui Cattle Co. beef. Good stuff, local and sustainable; pastured upcountry, then finished on pineapple sileage at a small feedlot in Waikapu. Yum.