I would suggest Berlitz, located on Ave. Estrella Sadhal?, across the street from the Government office building (Hualcalito), and close to the corner of 27 de Febrero.
The PUCMM also has a Spanish for Foreigners Program.
I have visited ALPI facilities on San Luis street. Not my cup of tea, frankly.
There are several other places where you can get personal attention at a much better price...
After all, this is pretty much a "total immersion" situation until you find a gal that speaks English.
Go for it.
HB