Minecraft wiki entry for Superflat_survival, associated text: Kelp
Sugar Cane: used to make sugar and paper.
Mushroom
Moss Block: You can get Moss Carpet, Azalea and Flowering Azalea; but you need to get dirt first.
Small Dripleaf: Don't buy it, there is no way to get clay in this world (no mason villager).‌[until JE 1.19]
Beetroot Seeds
Melon Seeds
Sea Pickle : You need to buy a coral block.
Cocoa Beans:‌[Bedrock Edition only] You need to buy a jungle sapling.
Other Saplings : If you want another type of tree farm than what currently you have, then buy different sapling from wandering trader.
Pumpkin and Pumpkin Seeds: Recommended to buy pumpkin block than its seeds, because both cost one emerald and you can craft 4 pumpkin seeds from a pumpkin.
Cactus: Require to buy sand or red sand from wandering trader (recommended to buy regular sand, it gives 8 sand unlike red sand which is only 4).
Fern