Here is a webpage that tracks the availability of the three different Apple 3G iPhones by apple store with links to the stores in each state. It is updated every 15 minutes.
There are also graphs of the percentage of stores with available Apple iPhones. It appears that around 10AM-noon each day there are bursts of availability as deliveries are made. Availability has been improving since Monday July 21 and the supply issues could be mostly resolved by Friday or early next week.