Just to chime in here, I had a similar need when my son asked me what to do about his 1960's era double oven where the glass in the door had broken. After some research I concluded he needed to replace it with tempered glass, which is harder, safer and more heat resistant than standard glass. I pointed him to a local (to me) company called Peninsula Glass that takes online orders and ships custom made tempered glass, but he ended up finding a company that was local to him where he could buy what he needed.
So my advice here would be to use tempered glass; if it's good enough to use in an oven then it is definitely safe to use in a popcorn popper. If your original glass is broken you can tell if it was tempered glass by the way it breaks; standard glass breaks along distinct lines emanating from the impact point, but tempered glass shatters into a pile of pea-size pebbles if broken.
Of course, it's not exactly cheap to have custom tempered glass made, but it's not prohibitive either. I've personally used this company a few times in the past and can vouch for the quality of their work and their integrity as a company.
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