I've answered alot of questions about this one.
First off, as far as gas masks go, what style/nationality/etc. are you looking for?
Do you want it to be a working model or replica?
You're gunna want shatterproof eyepieces/lenses because theres nothing worse than your goggles shattering into your eyes. One thing with working gas masks is that they can really restrict your breathing and they are HOT!