Aperture Science Portal Gun Themed Gas Mask
An artist going by TwoHornsUnited on DeviantArt has put together a Portal-inspired gas mask.
The gas mask is designed with the aesthetics of the Aperture Science Portal Gun and is fully functional as a gas mask in the event that the Combine invade Earth and decide to unleash a gas attack on the whole lot of us.
TwoHornsUnited intends to sell the mask to cosplayers and gas mask enthusiasts (yes, that's an actual thing) at the upcoming Metrocon convention. If you can't make it to the convention, you can message him directly through DeviantArt and order a mask for yourself.
The mask will be available in June and will set you back a hefty $280. Not a bad price to pay for a high quality gas mask that looks like something Cave Johnson would've come up with.