Actually, your best bet is to watch for when a store or a chain starts closing stores. You can buy there cabinets for a few dollars, and then fix them up. My brother picked up an old Nintendo cabinet, refinished it to get rid of the red and yellow color scheme and now uses it to display model trains.
As for doing it yourself. Yes, you can go to a glass cutter's and have the doors and shelves ordered. Don't ask for the display cabinet parts - ask for the dimensions and explain your intent. Usually, they can give you a good result. The rest of the frame can be assembled from standard lumber and finished as you like.
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Paul Grzelak
200GB with 48MB RAM, Illuminated Buttons and Digital Outputs