Sapphire Bundles are special packs offering not only Sapphires, but other valuable Items such as: Construction Boosts, Units, and Resources. They can occasionally be purchased at the Bank. If you receive an offer and make the purchase, you can find the various Items from the Bundle in their respective tab in the "My Items" window.
Bundles are offered on a random basis. Thus, whether or not you are invited to purchase one, and which Bundle it will be, depends solely on chance.