Hi. I was in the same situation (tourist) and wanted a third shot (the first two i had in Europe a year ago). First we applied for the booster via the official webpage. However, it was very difficult to have somehow the first two shots registered into their system. In the end what i did: i just told them that i was not vaccinated until now and i got a (Moderna) dosis. That did it for me. (As far as Argentina is concerned, i have one vaccine only. if required, i can show anyone that i have three dosis; in my case i didnt do it for travelling purposes though)
What i would do: just pass by a vaccination center and tell them that you could not register yourself (vacunar, or whatever the application is called is country restricted). Chances are that they will let you have the vaccine even without registering.
regarding Moderna: in calle Bolivar (edificio Historiador) they have Moderna for sure. In other places i dont know.