Hardened Felglass is a newly added engineering Part in the patch 7.1.5, which mainly used to crafted the 4 brand new goggles：
To have a chance to loot this item, you must have Boon of the Builder applied to your shoulders. When 7.1.5 went live today, Hobart Grapplehammer - the main engineering supplies guy in the Dalaran engineer shop - offered a "hi here you go" quest that gave you one of these. He also had the boon for sale to be applied to later gear.
Sprocket Containers can be Looted by players with the Boon of the Builder enchantment. These can contain Hardened Felglass at a not-very-grand drop rate, as well as crafting reagents such as sniping scope and randoms like pump action bandage gun. Most usefully, they can contain the schematics for the level 880 Skullblaster goggles. This is the only way to obtain Hardened Felglass as far. Fortunately, you can buy Hardened Felglass with wow gold
from the Auction House.
The newly made goggles's level is 880, and you could also trade via Auction House with gold, and the requiement is engineering level 1.
The Hardened Felglass drop rate low, that's why the price has been high.