I always call ahead to verify availability of items.
I also need an extra cable, went to one of my local Apple stores (I have 3) last Thursday afternoon, arriving about an hour after the last USB cable was sold. The day before there had been a wall full. I went home empty-handed.
Saturday afternoon I called the same store, was told they were still sold out and that a different store had cables on Saturday morning. I called that store and they'd sold out that afternoon. No joy on Saturday, either.
The first Apple store is expecting a shipment of items tomorrow (Tuesday) and I plan on calling to see if cables are included in the shipment; they never know shipment's contents ahead of time. Maybe the third time will be the charm.
The point is that this particular item is in high demand EVERYWHERE and supplies are being distributed to all geographical areas in order to be fair. This temporary shortage exists because nobody can use their existing cables for the new iPhones & the situation, while annoying, will be corrected as demand decreases because users are eventually finding supplies.
Patience is required and that's the way it is. Get over it. You're not the only one.