I consider myself fluent/bilingual. I am a native English speaker with bilingual level Spanish. I have worked as an interpreter for conferences at La Rural in BA. Send me a PM if you are still looking for someone!
If it´s the same when i did it a year ago you go to the Colegio de Traductores Publicos on Corrientes 1834 and ask for the English language translators "binder" and there is a huge excel speadsheet of all the licensed people in Capital Federal. Just find 5 or 6 numbers that are close to your house and call them and ask for rates. I can´t remember how much it cost me to do it last year (passport, birth certificate + apostille).... I would say 100-150 pesos is about right. I think taking it to a law firm would be more expensive because in the "binder" method you generally just go to the persons house.
Remember after the official translation you need to go back to the Colegio Traductores on Corrientes and get the documents legalized. That is fairly quick and easily, I think they do it while you wait or you can leave it behind (it takes forever) and it´s cheaper.