Some US states, such as Alabama and Utah, have no iPhones of any type in stock, according to Apple's data, while others are suffering limited availability of some models, notably the 16GB handset.
However, the company said that the 8GB basic iPhone is available in all stores.
For UK buyers, a message on the Apple Retail Store site advises that the iPhone 3GS is “likely” available in shops.
“Shipments of iPhone 3GS arrive most days and availability is updated nightly,” the site notes. “Return after 9pm to check iPhone 3GS availability at your local Apple Retail Store.”
This is not the first time that Apple has rolled out a web tool to save customers the trouble of visiting an Apple store. Last year a similar service was available, updated daily, but now the service has been enhanced.
"You can check the most up-to-date availability right here," says a posting on the US web page. "Shipments of iPhone 3GS arrive most days and availability is updated hourly."
Meanwhile, French technology blog Nowhere Else is reporting overheating problems with the new handset. The site shows a picture of a discoloured iPhone that it says came from using 3G services.
Other reports have said that overheating is a problem, particularly with handsets using the iPhone 3.0 operating system.
Rolls Royce will use AI powered by Intel's Xeon Gold processors and SSDs for memory
The most extreme range of orbits yet observed in such a young star system, claim University of Cambridge astronomers
HP and Centrica are the first industry partners to sign up to the government's new Code
New ice grows faster but is also more vulnerable to weather and wind