There are a couple of sastrerías on Catalina González street between 5 de Mayo and Cuauhtémoc streets in downtown Zihuatanejo. My wife uses the one near the corner owned by a very corpulent woman. If you get lost or confused just ask at the tienda for the "mujer sastre". I don't recall her name.

We used her to change t shirts into tank shirts, and she did 60+ of them for us. We were very pleased with her work and prices.