You can't buy them all together in one set (as far as I know).

There are 7 dvd volumes in the series (3-4 episodes per dvd).

www.deepdiscountdvd.com has excellent prices. Though sometimes it is a bit difficult to tell exactly what you are getting (they get the titles right but they often don't tell you what is contained in the dvd set) so I would open up amazon as well and compare the two in order to figure out which dvds you want (there are two or three versions of the same dvd with varying bonus features and items).