I get most of my DVD's from
www.dvdsoon.com a Canadian site. They take a little longer to arrive, and if you order anything over £18 you have to pay vat and a charge to the post office too, but they are very cheap on there.
I am a member (about £20 per year, well worth it for me) so I get free postage and a small discount on everything, plus I get points that I can save up to get things on there.