Oh, I don't know, team's site might be a start: http://dcunited.mlsnet.com/MLS/dcu/load.jsp?section=about&content=tradition K
This probably has the most comprehensive list of all United foreign games http://fujita.atmos.colostate.edu/~john/homepage/ scroll down the left hand menu and find DC United foreign games.
Yeah, the United and MLS sites can sometimes be lacking in certain things. They can sell you a half dozen things, but they can't get you an all-time fixture list.