You'll not be able to get the key working again if you dont have the white scissor clip that goes underneath. You can often buy keycaps on their own. But you'd need to identify which version of clip you have, as there are different versions depending on the manufacturer at the time.
if its one of the aluminium keyboards, then it will usually be either Type F or Type H.
(these are not identified by Apple, as they didn't start providing keycaps until 2015. We've come up with our own identification charts.)
here is a link to help identify which type you have.
If you then click on the type you have, you will be taken to the listings for that type. We sell every key with a white scissor clip, so you can replace both if need be.
http://www.thebookyard.com/apple-keycaps-clip-select.php?cPath=66_98_102
I hope this helps.