I find iStockNow to be a few minutes behind the actual stock levels, which is fair since its third party. After my store were out of them (according to apple.com) istocknow still showed available for a few minutes afterwards. That said, It can be useful, as thats how I noticed they were available when I got some, however if you want the best possible chance its better to keep refreshing the Apple Store page where it'll change to "available Today" immediately once stock is receipted into store.
Have to say, the popularity of AirPods have taken me completely by surprise. I didn't bother preordering since I figured they wouldn't be that big a deal (both in demand and how well they work). Happy to say I was wrong about both.