The two most popular brand name cases out there are Amaray and Alpha. Both brands have single, dual and multi DVD cases. I like iNetDVD because they not only have the standard "black" cases but they have all the special color cases such as the lime green XBox cases, Blue PS2 cases, white Disney style cases, silver ID4 style cases and whatever elese you might like.
I first ran into these guys through their eBay store and have been shopping with them ever since. Check'em out:
DVD Media Cases at iNetDVD
iNetDVD currently has two quad cases for sale, an Alpha black and an Alpha white. The white version is currently out of stock (10-24-2004):
Alpha Black Quad DVD Case
Alpha White Quad Case
I hope that helps.
-BadAsh