I'm with Tom. I wouldn't stress too much about it. Old Sol puts out a fair whack of UV, but so do HPS lights. My 1000HPS lights will bleach, make brittle or disintegrate non-UV stabilised plastics in my op in just a few months.

Some modern window glass does have various UV & IR filtering characteristics, but you generally pay extra for it. All the same, I'd avoid using glass in an aircooled hood that was specifically labelled as blocking UV.

If you buy a cut pane of plain ol' common cheap window glass from the hdwe, you're not apt to get anything you're not paying for.

HPS tube envelopes are usually borosilicate glass, which is about 95% transmission efficient in the UV end (250nm), dropping sharply in transmission on the IR end. Many people are making cooltubes out of Pyrex Bake-A-Round tubes, which if made before 1998, are also borosilicate glass. "Vintage" Bake-A-Rounds are commonly available on Ebay.

Common soda-lime glass is no slouch in passing UV, about the same as borosilicate.