I am going to Oktoberfest for the first time this year as well (9/25-26) - our reservations were purchased by a friend of a friend who lives in Frankfurt. Our package was EUR 98/pp and includes seats at a table in the Hofbrau tent, one night hotel, breakfast, vouchers for food and 2L of beer, a Munich guidebook, and an "Oktoberfest hat" (whatever that is). We booked this in March. Unfortunately I don't know where he went through to get the reservations.

If you don't need hotel or a package, I believe the only way to get a reservation for seats in the tents is to contact the individual tent directly - see the Augustiner portion of the official Oktoberfest website here for contact info. Some of the tents have websites but I don't see one for Augustiner. For example, the Hofbrau site shows what dates and times are free and then you can book by fax.