Register with listicket here http://www.listicket.it/?lang=EN&S= and buy the ticket in advance. San Siro is never full when Lazio play, but just to be sure.
And I don't know how to get the tickets for the away sector specifically.
I'm also attending this match, although I'm having second thoughts. The problem is the only way to get there is in a bus with 15 milanisti from Croatia and I have to travel "incognito". Other problems include we haven't won against Milan for 13 years, it will cost me a lost of money I don't have, and I have a short fuse (bad temper) so there's a great chance of a beating.
The plus side is it's cheaper, the ticket issue is sorted in advance and FUCK MILAN LA LAZIO IS MY LIFE
"One love, One Lazio SUPER TECH!!!" - mr douglas