Store Tokens can be purchased at any general store in Andris, Lerilin, Marali or Mirith. They can also be purchased at the Silver Leaf Shop (which requires a Silver Leaf to gain entry.) It has been known for Store Tokens to sometimes be found in chests in dungeons.
Each token costs 50,000 gold. Should a player feel buyer's remorse over the purchase, you can sell the token back for 50,000 gold.