Not sure about leaves but I've purchased fenugreek seeds (fenogreco in Spanish) at an Indian spice shop on Cuidad de la Paz in Belgrano. I can't remember the name of the place but I think it's between Hernandez & La Pampa. If not, it is very close and on the Cabildo side of the street.
It sounds like you're interested in (South) Indian cooking so you'll find many useful ingredients there even if they don't have fenugreek leaves (a minor ingredient in most recipes). If you're looking for anything Indian, this should be your first stop.
As far as ginger and garlic paste are concerned, don't buy them. Find a ginger grater in Barrio chino instead. Mine (brought from the U.S.) is metal, which I think works better, but I've seen ceramic ones too. Essentially it is a flat surface with raised bumps that quickly make a paste of ginger, garlic, onion etc. It is one of my favorite kitchen tools.