I get mine from
www.ebuyer.com.
You can get 500GB 2.5" SATA drive for about £45, add an external case for about 6 or 7 quid & you're good to go, just plug it into a USB port on your laptop & you have another drive (though you might need to format it forst).
I usually get 2 or 3 at a time & use them as storage & backup for my photos.