If you are interested in purchasing an iPhone 3GS, but are not sure about whether your local Apple store has any in stock, then the Apple iPhone 3GS availability Web tool is for you. According to the page:
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.
You can access the tool to check availability in your state by clicking here.
If you live in Texas, your chances of getting an iPhone 3GS are very good as you can see from the example below.