At the moment the best tournaments taking place are part of the Argentina Polo tour. There is some 22 goal coming up this weekend which is pretty watchable, though most of the real stars are now playing the US, Spanish or US seasons. You don't need tickets (free even for the matches at Palermo) and there have been some suprisingly big crowds. For details of matches and venues look at www.argentinapolotour.com. More general info about what is going on is available from the argentine polo association. www.aapolo.com . If you want a crack at hitting a ball yourself, there are lots of decent places within an hour of the capital.
AFAIK - the season is all bout over (in terms of high goal) as the majority of high-goal players are over in the US for the Open and then off to the UK. But there are always low goal games going on.
The only matches you would need tickets for are the Opens (Tortugas, Hurlingham and Palermo) in Oct-Dec.
Miles - do you know if they are playing the International Tournament again this year at Palermo? Last year it was Arg, Brasil, some other country I'm forgetting and the "rest of the world". But I haven't seen anything about it this year.