The military surplus store is your friend. The one in that episode is of course a vintage WWII English one, but I bet a modern one would work on the creepy effect just as well.

Oh, the one we have is actually an Australian version of a Swedish brand called AGA.

It was originally developed for fire fighters, but can also be converted to a diver mask. We have a bag for a WWII one, but the rubber doesn't usually last even though it is rubberized canvas. You may get lucky though and find one online that someone has had just sitting around in a good temperate environment.
Any view of things that is not strange, is false.