Big Island Fishmongers?
Believe it or not, all of the grocery stores will have great fresh fish. Do I presume you are staying in the Waikoloa Beach Resort area as opposed to Waikoloa Village? If so, there is Da Fish House in Kawaihae which gets the daily catch and really nice people working there. Take 19 and turn left at the fork; go around the corner on 270 (toward Hawi) and it will be on your right. There is Island Gourmet grocery store in WBR and another grocery at Foodland Farms in the Mauna Lani Resort complex.
Enjoy! (And don't forget to get some Best Foods potato salad)
I found the supermarket fish variable: we got some good fish and some great Kaua'i prawns at the supermarket in Waikaloa Village, but the market in WBR had only a very small and *very* expensive selection, while the Foodland in Hilo only had dried cod and poke, unless there was a special section we didn't find.
Thanks for the tip on Da Fish House: next trip, maybe.