The external cases that I use to add HDD are from Welland. I buy at wholesale but their home page is:
Just Google/Yahoo or telephone your local computer shop, they should be able to supply similar or the same items. You have the choice of 2.5" HDD (Notebooks) or 3.5" HDD, and with the 3.5" the choice of SATA or IDE connections to the HDD in the case.
There are also the prebuilt External HDD from Maxtor, LaCie, Aldi Supermarkets (here in Australia anyway), Seagate, and others that can have USB 2 and Firewire 800 connections. The main benefit of these is they often come with backup software (for Windows anyway) to enable easy file backups.
Hope this helps a bit more.