During the bazarr in Argonia there is a shop that will sell Yggdrasil leaves, but they will only allow you to have one and won't sell another unless you have zero leaves. The game cannot detect items in your alchemy pot. So you can trick the game and stock more than one leaf. Buy a leaf and TEMPORARILY combine it in your alchemy pot with some magic water or some red and blue mold. Then you simply buy another leaf and abort the alchemy pot process and reclaim your first leaf.