Lunengard was a big city with lots of shops. It was possible to find anything there, and a potion wouldn't be different. The problem was not finding a shop with potions to sell but rather finding the best shop with both quality and price.
The luxurious places had higher prices, but everyone knew that the quality would be top-notch, and that was the most important thing to look for when buying a potion.
No one wanted to drink a low-quality potion. The impurities would make the potion way less effective, and the chance of getting sick because of it is high.
So, in the end, Kai decided to go check the luxurious shops. Not only because he wanted a product with high-quality, but because he wanted contacts.
The most rich, influential, or strong mages visited the luxurious places. Kai went to a place called Royal Potions.