The short and straight forward answer - Buy new one(s).
A long answer? May be you insure them and go through the process to get new ones (I don't know if there are companies out there for that, may be, who knows...?) because Apple's AirPods for your iPhone 7 are just like any other Apple accessories -- there's no anti-theft measures in place to protect your shiny wireless earbuds.
If one is lost or stolen, Apple says you'll be able to buy just one (but didn't say how much it would cost to buy just one.)
AirPods costs a whooping $159, so be careful with them.