Scott Health and Safety Products NTC-1 CAN industrial/commercial NBC filter with the 40mm NATO thread offer the same protection as the other military gas mask filters on this site.  Sealed in manufacturer’s original packaging, these retail for $49 each, but these out of date filters can be had at substantial savings! 

This sale is for ONE filter, not two as shown in the main photo.

http://www.approvedgasmasks.com/approvedgasmasks.com/gas-filters.htm