The Kongregate Sticker Marketplace is where you can purchase new sticker packs to use with your friends in chat. You can get to the sticker marketplace by going to Kongregate.com/stickers
In the marketplace you can see which packs are for sale. It will be updated on a regular cadence to introduce new stickers that you can purchase.
When you purchase stickers in the marketplace, there is a chance the pack you purchase will be a rare shiny pack.