Since iOS 10 the Messages app has a dedicated store for sticker packs and games, how do I access it?
To get to the iMessage App Store, open up the Messages app and open a conversation. To the left of the text box you will see and icon that resembles an "A." Tap on that, then tap on the four oval icon in the bottom left. Now you will see a blue "+" icon, which is the store. Tap on it to go to the store.