My entrance door has glasses. I'm in need of replacing the glass part of the door. I need to know where to buy glasses?
Since you are on Stack Exchange, I can safely assume you have access to the internet. Use Google to search for 'Glass Store', and in its local search it should find some in your area. If it doesn't, go to any hardware store and ask if they can order glass or cut glass to fit. If they don't, ask them where you can go.
It's true that a glass shop can help you, but be sure to let them know it's for a door. Building codes require better glass in doors, and even nearby windows. I don't recall if the right term is "safety", or "tempered", or what. The number of kinds of stronger, safer glass is rather surprising, so the glass experts can tell you what's right.