Kudos and odd question

Check at the Saturday morning Eco-Tanguis market behind the Museo. I think you'll get some good leads there.

If you don't want to wait until you arrive in Zihua, you can contact these two vegan restaurants beforehand. They will most likely be able to point you in the right direction, if not give you the info you need right away.

Cooperativa Ecovegana de Zihuatanejo: https://www.facebook.com/ecovegana

Entre Zankas: https://www.facebook.com/EntreZankas/

Cooperativa Ecovegana de Zihuatanejo has a booth at the Saturday morning Eco-Tanguis market.