The Teatro Colon box office is currently selling subscriptions for the 2016 opera season. Sale of single tickets has not yet opened, and usually does not start until close to the date of the first performance of the season. That being said, I have not had problems in the past buying tickets after arrival in BsAs. Setting up an account and buying tickets online is another possibility once single ticketing opens. There is a dress code of sorts for the first performance of each opera (5 April for DG) for certain sections of the theater but I don't know how strictly it is enforced.
BsAs has another opera company - Opera Lirica - that opens on 8 April with Faust, but it looks like that date doesn't work for you.