Apple does offer a replacement battery service to cover the concern of a dead/weakening battery later in the systems life. If you want to go further and not rely directly on Apple, there are a number of places that sell laptop parts like iFixit. The new Air battery looks a little different, but should be replaceable yourself with just a few screws off the bottom panel. iFixit should have a complete teardown guide in a few days for the new units.

If your concern is more about in the field runtime, HyperMac sells external batteries and a charger cable for the MacBooks. However, they are currently in a lawsuit over this with Apple, and will be ceasing sale of the charger cables on November 2. They are in discussions with Apple to become a properly licensed vendor, and hopefully it works out. Apple has allowed similar battery extenders to be licensed for the iPhones, and even sells them in the Apple stores, so there is some precedent for this turning out well.