Good luck, let me know if there are any questions, but hopefully you'll see what I mean once you start looking through their websites, they do make the distinction between drives and their intended design purposes reasonably clear.
I suspect it will be largely down to what you can get your hands on from places that sell to private individuals and what they have in stock, although I would hope that hard drives should be fairly universally available as they are a commodity item.
Some of the newest drives only launched in December 2014 might still be a little hard to get hold of, so if you want cutting-edge you might either need to look at sourcing from mainland Europe or just place a backorder and wait a couple of weeks (if there's something in particular you've got your eye on, I might be able to get you an idea of what the wait time is likely to be if your chosen e-commerce site doesn't provide that information already).