on my route in, they are usually around the North end of the stadium east of the Armory/Price hall
i've also had people hand me free tickets as i was walking past where the tour buses drop people off on Asp, on the east side of the stadium just north of Lindsey
people are usually selling on Campus Corner at Boyd and Asp or Jenkins as well, but they always seemed more high end prices
but really you can just walk around and look for people holding up tickets. try to find a regular guy holding an extra vs. some professional looking guy with a stack of them