Do you mean the fixtures themselves or the glass shades? The glass shades you can get just about anywhere. The house I grew up in (ugly 1950s ranch house with ugly 80s and 90s additions) had them on all the ceiling fan lights, and I know I've seen them in  hardware stores too.

As for the actual fixtures, I don't know about that. They may have been custom made for the mansion (some of the wallpaper, like in the WDW foyer until recently, was made custom for the mansion so maybe), or long since discontinued. You may have to settle for more generic faux-historic fixtures, or genuine gas/electric fixtures from the late 19th century or early 20th.