Where can I get a gas mask in Mauritius?

Try your local pharmacy; if they don't have it in stock then ask them where you would be able to purchase it.

Otherwise there's this company, Ducray Lenoir, which does medical equipment. Give them a ring to enquire.

PS : Please don't answer your own question unless you've solved your own problem and want to share the solution with others. Sometimes it takes time to reply to someone because we don't know the answer yet. So be patient.