The short answer is: nowhere. The iPhone 3GS, introduced in July 2009, is only available in 16GB and 32 GB models. An 8GB iPhone 3G is still available new as of this writing.
Some rumors had appeared of Rogers Wireless offering an 8GB iPhone 3GS in Canada. These reports were based on iPhone comparison charts on the company's website that showed the existence of an 8GB iPhone 3GS.
