You can find the Silver Crate purchase option by tapping on the "Shop" button at the bottom of your screen and then selecting the "Crates" tab at the top of your screen. You receive one free Silver Crate every 10 minutes, up to 5 free Crates a day, and you can also purchase more with bucks.
Silver Crates are a great way of obtaining certain Hero Fragments and Gear used for upgrading your Heroes. Each purchase provides one reward that is randomly selected from the Crate. You can view the list of available rewards by tapping on the picture of the Crate. Please note that this list doesn't contain all possible awards, just the highlighted rewards, and may change from time to time.