Messi has 15 goals and 12 assists in 53 games for Argentina NT. I can send you a full breakdown game-by-game if you want.
Try this website - http://www.lionel-messi.eu/stats/ It tries to give, by season, the goals and assists of Messi for Barcelona as well as the Argentinean NT. Doesn't look entirely accurate (only 12 goals by Messi for NT counted in there), but its a start.
