Look in your classified telephone directory under headings such as "safety equipment" or "safety supply." In the larger cities you will probably find one or more companies that sell this sort of equipment to the industrial market. They may not have brick and mortar shops, because they sell through sales people who visit their customers.
Ebay?
Ebay
ebay
Military surplus store, Ebay.
Lowe`s, Home Depot and I get mine from eBay.
toys r us i got mine from them just ask the people
You can purchase them on-line and have them shipped to you.
I'd try Ebay. If you buy a plain mask you can draw the stitches on yourself.
Buy better air filters. Wear a face mask if you're paranoid. Have fun.
ebay
Ebay / Amazon
I'm sure Ebay will have some Spiderman masks listed.