Q: Why does it say “No network” or "Unavailable" in the in-game Store next to the item I want to purchase?
The text of the "No network" or "Unavailable" message is somewhat inaccurate; what it really means is "the game cannot connect to the iTunes / Amazon / Google Play servers", which could possibly occur even if your device does have an internet connection. Until the connection is established, you will not be able to purchase that item.
Unfortunately, there are a number of things that could create this situation, not all of which are under your control or ours. This page contains some things to check and/or try.
Once the "No network" or "Unavailable" message is replaced with a price, clicking the item should launch your device's App Market app and let you make the purchase.