La Viruta Tango Club has salsa lessons on Friday I believe. I haven't gone to the Salsa ones yet, but I've gone for the rock and roll and tango classes which I enjoyed. For those two they have days that the lessons are free.
Something you might want to consider before taking classes is what style Salsa you would like to learn? Cuban, New York/Puerto Rican style on 2, LA.style on 1. If you know what city you will be dancing in that might help you decide what style to learn. You can easily find classes with a google search or on Facebook. La Viruta probably would be a good place to start. Lots of people for social connections. This club in San Telmo has some amazing Cuban dancers https://www.facebook.com/pg/buenosayresclub/about/?ref=page_internal. Don't know if they have classes. Thursday and I think Saturday they have Salsa nights. Tango on other nights.