They don’t “spawn” automatically. The starting items is based on the starting prosperity level. Draw the stack of cards nearest to the shop to the shop box. After prosperity thresholds are passed you need to att the stacks for the different prosperitz levels.

All piles are on the left side.

There is actually a hidden button to the right of the deck of face down prosperity stacks. If you click it, all of the cards in that stack will be laid out along the top of the board automatically, if you're into that. So in a brand new campaign, you can create a store by clicking the button for prosperity level 1. But you're going to have to search through decks and pull out cards manually as you loot treasure chests.