I've commented on it elsewhere, but the rarity on a lot of the crafting ingredients is definitely very wonky. It's just a result of a drop system that's so heavily reliant on RNG. I'm a hoarder by nature, so I've managed to mitigate it a great deal in my games, but it can definitely be a problem for some folks.
It's worth noting that the merchant inventories reset each time you go up a level, so make sure to go back and visit them to harvest any additional crafting mats or consumable items (potions, arrows) you want to stock up on.
