I found this GBA accesory lot on ebay. Its currently at 20US with free shipping, and it has the GBA carrying case! I believe it is a rare item, and the auction is about to end, so you have to be fast if you want to use this for a Wii Portable.
http://www.ebay.com/itm/Lot-of-5-Ni...457450?hash=item4b1d6136aa:g:qcsAAOSwguxZd~1O
Happy portabilizing!
http://www.ebay.com/itm/Lot-of-5-Ni...457450?hash=item4b1d6136aa:g:qcsAAOSwguxZd~1O
Happy portabilizing!