I just ran across 2 interesting sites for finding tartans. Well, actually two different pages on the same site.

The first is for finding a tartan by name: http://www.house-of-tartan.scotland....se/tfinder.htm

The second is the cool one. You tell it what the colors are in the unknown tartan and then it searches for matching tartans! That's at: http://www.house-of-tartan.scotland....se/reverse.asp

They also sell the tartan by the yard if you want to try your hand at making your own.