You can find the Gold 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 Gold Crate every 48 hours, and you can also purchase more with gold.  

Gold Crates are a great way of obtaining rare 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.