my experience is from the Maracana a couple of years ago, so forgive me if i'm off base:
http://soccernet.espn.go.com/fixture...ileiro?cc=5739
Not sure about buying them online, but how about just buying them at the stadium? Places like the Morumbi (Sao Paulo's stadium.... aliás where are corinthians playing these days?) are so gigantic that they always have tickets available unless it's a real 'clássico'/libertadores final.
If you know anyone with a student card, you can probably get a half price ticket... Noone ever asked me to show my student card on the way in