Pueyrredon and Juncal, near Hospital Aleman. Germans love ophtalmology. I've actually seen plenty of Acuvue adds around that area. Pförtner has them, and I believe so does Luzt Ferrando. The latter is usually less expensive, same quality.
Well, I go to Pfortner. They have offices around BA, but the main one is on Puerydon (I know that is spelt wrong) and Juncal, more or less. They are amazing. I am sure there are cheaper places, but they actually re-examined my eyes corrected the presciption my Opthamaolgst gave me, so they really take care. Top-rate professionals.
But not that cheap. I buy the weeklies and wear them for a month. Never had a problem.
If you are certain of your current presciption, I would just suggest you ask around.
i have bought them in a few places. the last place i bought them was at the Alto Palermo shopping center on the top floor. i can't remember how much i paid but it was during a buy one get one free offer. so i bought 6 boxes of 30 for about 500pesos (i think).