For TShirts Ona Saez has fairly good quality, but they fit small (ie you'll have to go up at least one size from your usual). Also, Bolivia in Palermo -- on THames at about Costa Rica, has some very nice men's things.
(From post I interpreted t-shirts because of brands mentioned... but the above will also have button downs, otherwise for dress shirts off the rack try Etiqueta Negra, Felix, Giesso, Key Biscayne. If you want to hit a mall, forget Alto Palermo, for higher end go to Paseo Alcorta they have all above brands less Key Biscayne I believe. )