I think M17 gasmasks aren't hard to find.
Here are a couple of links that might interest you and I'm pretty sure there are others out there:
http://www.regimentals.co.uk/viewphoto.php?shoph=55703&phqu=4" onclick="window.open(this.href);return false;
http://www.regimentals.co.uk/viewphoto.php?shoph=54413&phqu=3" onclick="window.open(this.href);return false;