Not sure if I'm just blind or perhaps this is intentional but.... it doesn't seem the Wizardly Tower in the Gobsquat has magical bottles available for sale. I looked around the Gobsquat, unless I'm just blind, it doesn't seem there is an NPC for goblinoid/shady PC's to get magical bottles anywhere there.
You're not blind, you just have poor Search rolls ;). There is a shop somewhere accessible to shady PCs without going to the Ziggurat proper.