Following our March report on 16GB iPhone shortages, new checks reveal that the supply shortage has spread to the 8GB model as well. Nine U.S.-based Apple retail locations contacted today by iLounge yielded only two locations with 8GB models in stock, seven locations that were completely sold out of iPhones, and not a single location with the 16GB model in stock. Previous reports from stores suggested that the shortage might last until May, and though…