Didymium filters are a not-uncommon off-the-shelf item sold by a bunch of manufacturers including Hoya. They are a notch filter, i.e. they filter out a very narrow slice of spectrum; the ones used for glass blowing filter out the sodium emission line from high temperature glass.
You can have the notch at different wavelengths, which means for photographic use you can buy red-enhancing, green-enhancing and blue-enhancing filters. B&H sells a B+W Redhancer, for the Hoyas I think you need to get on eBay.
And this dpreview thread contains probably all the info you need for photographing glass blowing.