Derailleur Hangers are the handy little aluminum devices that keep your derailleur in perfect position on your specific bike, and they're nearly as unique as fingerprints or snowflakes. OK, that might be an exaggeration, but if the specific one on your specific bike breaks during FANY, it can often be hard to find the specific replacement in the small towns we'll bike through. They're cheap insurance, usually $20-$30, so it's a great idea to go to your local bike store and order one now to bring on FANY. With any luck you won't need it. But if you do need it, you'll be back on the road 1 or 2 days faster.
FANY is a huge fan of supporting local bike stores, but here's an on-line option. We're not related to these folks, or even endorsing them over other online options, but it seems like they try to make it easy to order. And to prove my point, they list over 500 derailleur hangers