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.