The Headphone Adapter and Stereo Headphones designed for the Game Boy Advance SP are available for purchase - if you know where to look.
After various problems in locating the Game Boy Advance SP Headphone Adapter, they can finally be found for purchase. If you go to the Nintendo Online Store, under the section 'Parts and Accessories, Manuals' there will be a 'Game Boy' section. There is now a 'Game Boy Advance SP' section, which has the Adapter for purchase at a price of US$4.25.
Alternately, if you don't wish to use the headphone adapter, but still want stereo sound, there's a set of headphones specially designed for the Game Boy Advance SP. Instead of the regular headphone jack, they use the same jack as the headphone adapter, meaning that they can only be used for the Game Boy Advance SP. They are priced at US$19.95. If you order these headphones from the online store, you'll also receive a one year subscription to Nintendo Power.
Both the headphones and the headphone adaptor are only available from the Nintendo Online Store. The headphones will start shipping as of the 31st of March, but it seems that the headphone adapter will be shipping immediately.
