Play sell them for £17.99 but its out of stock there to. They do have some you can buy from Playtrade but the prices are a rip-off unless you got a second hand one which wouldn't appeal I'd amagine.
http://www.play.com/Games/PlayStatio...t/Product.html
£20.95 @
http://www.mymemory.co.uk/Sony-PS3--...xed-%28PS3%29- but I've never used them so dont know what there like.