can you buy tickets before matchday at Baleteria (boxoffice) outside of the San Siro or the Banca Intesa?

is true everything is closed on sundays in milan?

Banks are closed on Sundays, yes. But tickets ARE available at the ticket office outside the San Siro on gameday (there are usually lines so go early).

Also, if you're from a country other than Italy, the club is selling tickets on their website again to foreigners.