Mostly, games are played during weekends. There are some popular teams such as River Plate (which has descended from the "A" category and now plays in "B" national category) and Boca Juniors.
Depending on how popular teams are, it gets harder to get the tickets.
I would recommend that when you get to your hotel, ask the personnel at the counter, who surely will lead you to people of tourism agencies.
I have googled this for you and found this interesting site, maybe you can make it online ;)
Good luck on your trip,
Gimena about 8 years ago