Okay, so that much is obvious, but it never hurts to buy a spare cable either way — especially since Apple’s own model isn’t exactly known for its durability.
Personally, I use this Luxe Series cable from Monoprice; it’s not the cheapest, but it feels higher-quality, is MFi-certified, and has a nifty little LED light that tells you when the phone is fully charged. It’s worked without fail for several months now.
If you do buy a longer one, just know that those tend to charge slightly slower than cables with less distance to cover.