In Palermo you may look at glance to VIVERO (called Kadan) of Jardin Japones (Japanese Garden) , and if you can wake up very early, go to MERCADO DE FLORES (FLOWER MARKET) in Olavarria 3240 Barracas,Capital, you can go until 9 oclock (maybe from 6 am....= or on Thursday until 11 oclock, only on Saturday from 9 to 12.
There get together big suppliers of the flowers and green plants not only for the city and the provincia of Buenos Aires but also to the around Argentina.They are the best, freshest and cheapest in the country.