Check ebay. I bought mine, a white one, there for £7 inclusive shipping.
http://cgi.ebay.com/New-Nintendo-DS-Lite-P...1QQcmdZViewItem
But I would rather buy the one with a magnetic seal instead of the one above.
this one!
http://cgi.ebay.com/NDS-Lite-Leather-Carry...1QQcmdZViewItem
didnt...