Best answer: If you lose a single AirPod, you can purchase a new one from Apple. From there, you can pair the new AirPod unit to your existing one. However, the replacement must match the generation of your current AirPod.
Yes, you can
If you can't find one of your AirPods, Apple does allow you to order a replacement for $69. You can often find replacement AirPods being sold on eBay for less. The company will also sell you a replacement charging case for $59. Apple also sells the 2019 wireless charging case for $79 that works with both AirPods versions.
Apple AirPods vs. Apple AirPods 2
Because the original AirPods and second-generation AirPods use different chips, you can't match one with the other. In other words, a W1-powered AirPod won't work with an H1-powered AirPod 2. When ordering your replacement AirPod through the Apple website, you'll be asked for the serial number which is located on the original Charging Case that came with your AirPods. With this information, Apple will send you a compatible AirPod unit as a replacement.
Did you really lose it?
Before spending a dime on a new AirPod, try to locate it using the Find My iPhone app for iOS. With the app, you might be able to find the missing earbud as long as you're within Bluetooth range. If that doesn't work, you can think back to the last place you had the AirPods and maybe you can find the missing one there. At the minimum, by backtracking you might be able to get close enough to the missing AirPod to activate the Find My iPhone feature.