Slings are in the design files, but not on any merchant/loot lists, so they never show up. If you are on a PC, you can use this mod:https://www.nexusmods.com/baldursgate3/mods/58
It will open up slings, along with a lot of other weapons and armor.
I have tested it in patch 4 and works, but only because it is a bunch of loose files, not compiled in PAK format. Patch 4 completely fubarred PAK-formatted mods - not only do they not work, but they also mess up your hotbars.
Fun note: That mod also tweaks the character starting gear, but since it was designed before the druid came out, a druid starts with nothing - no inventory, no clothes, nothing. Fortunately, it is fairly easy to find armor and the nautiloid is littered with weapons. And hey, who needs weapons or clothes when you have Wild Shape?