Just check Craig's List.
One of the best ways to get tickets is to find a student ticket, because usually they are cheap (unless it's a big game) and all you have to do is pay the student and then get it validated. Usually, it can be much cheaper than other tickets.
Getting it validated means you have to pay the difference between student ticket value and actual ticket value, so in addition to paying the student, you'll be paying the rest of the normal ticket price to the athletic department.
Yes, but I'm referring to the fact that students don't usually ask outrageous prices for the tickets. Many sell them for the $21 student value or just over, and then you pay the validation fee to make them the normal ticket price.
However, buying tickets from season ticket holders who are selling them can often be very pricy. For example, if a student sells his ticket to you for $21 and then you pay the validation price, it will be around $60 spent. However, many non-students sell their tickets for maybe $100 or more.
Now, that practice may not be entirely legal, but that's how it has been in my experience.