Crayfish, from the fishmonger in Upper Limsa. They stack to 999, so I only needed 10 slots. It might be better to just breakdown dungeon loot, as it doesn't boost Culinarian Desynth past 60, and it's *really* slow (~45min per stack), but it's good for afking between queues and it's cheap.

Makes sense. If you have a little bit more cash to spare, keep your eyes open for fish on the market place. They sell for dirt cheap when they aren't used for turn-ins. Sometimes, you'll get something other than sand, like... a bag of coins worth 5 gil!