Apple has posted a Web-based tool which allows potential iPhone 3GS customers to check on the availability of the devices in Apple Retail Stores.
Apple page states:
The Apple Retail Store likely has your iPhone 3GS in stock. You can check the most up-to-date availability right here. Shipments of iPhone 3GS arrive most days and availability is updated hourly. The 8GB iPhone 3G is currently available in all stores.
Select your state and then check the list to see which Apple Retail Stores currently stock iPhone 3GS.
For example, choosing New York returns a chart this:
[Attribution: MacRumors. Thanks to MacDailyNews Reader “James W.” for the heads up.]