Finding in which expansion comes that card you need

The Collection screen not only shows the items you own. You can use it to check what comes in each expansion.

But if you want to know which expansion brings that card that you need, you better go to the Library and search for the card you are looking for. Each card comes with the list of expansions it comes in.
You can even tap on the expansion name to go to its own screen and check what else comes with that expansion!