If you are in the USA and trying to buy an iPhone 5s, head over to http://iphone-check.herokuapp.com/, enter your zip code, cellphone carrier and pick a color, and then it will show you availability at Apple Stores "near you."
I used the site to find a Verizon, 16 GB, Space Gray iPhone 5s in my area. I placed an order for in-store pickup, and about two hours later I was walking out of the Apple Store with a new iPhone.
Two caveats: First, "near you" is a relative term. The site seems to always show 20 Apple Stores, but some of them might be quite a distance from you. For me, the last store on the list was about 350 miles (and nearly a six-hour drive) away. Each result is linked to an Apple Retail Store, so if you aren't sure where it is, click on it to get the address. Second, availability can change fast. My local Apple Retail Store said that they were only filling orders through http://store.apple.com, which had been designated as "in-store pickup." The good news is that I knew I had an iPhone waiting for me before I got into the car. I definitely recommend using that method rather than driving to the store and hoping that they still have what you want in stock.