The Airgas Brand filter lens, is a heat-treated glass filter plate often used for welding protection. It measures 2” x 4-1/4” and is available in various shades, such as Shade 5 as pictured. Prices for this item vary, typically ranging from $1.50 to $2.25 (each this is a package of (5) )depending on the source. It can be found on platforms like eBay and specialized tools retailers   .
eBay