I tend to use ebuyer.com - but there's many more general-purpose component stores like them. Amazon usually sells these too - although is rarely competitive on price. There are a few dedicated memory stores, where the memory stick price will probably be the lowest - however, they are unlikely to sell graphics cards.
Quickest way to find a lot of stores? type "memory stick" into google, choose one of the sponsored links, or one of the price comparison links.
SD