All internal HDD's for desktops are 3.5". It is a form factor standard, (laptops use a 2.5" standard - smaller more spendy).
SO what you need to buy is a 3.5" HDD of what ever size you want.
Like this one for example:
http://www.newegg.com/Product/Produc...82E16822144338
Then buy a External HDD enclosure for a 3.5" drive like this:
http://www.newegg.com/Product/Produc...82E16817145162
or
http://www.newegg.com/Product/Produc...82E16817609002
but it is fanless so can cause issues...
And slap them together, Voila

Compared to :
http://www.newegg.com/Product/Produc...82E16822144385
Now there are cheaper ones for sale but when it comes to HDD's I dont cheap out on no name manufacturers.
Remeber prices all vary as do materials and colors. Also make sure you read the interface type. Like the one I posted uses USB to connect. But they are very simple and can very cost effective compared to buy one "Already assembled" for more money and as one stated earlier internals sometimes have better warranties.
Reb

PS I just used Newegg.com for an example, shop around and might find better deals.