Curry leaves in Mexico?
No guarantee, but you might try the Mercado San Juan, on Calle Ernesto Pugibet in Mexico City. There are Asian food stores adjacent and inside the market are Asian produce stands...and much more. We visited the store, "El Dragón de Oro" next to the mercado. The selection was limited, but I still found some usefull items.
Be sure to look inside the mercado.
Even if you don't find curry leaves, it's worth a visit.