Here are three sources for domes I have come across:

    The Glass Dome Co.
    Phoenix Farm
    Mattishall
    Norfolk NR20 3RH
    UK
    PH: 01362 858480
    (round, oval, antique)

    Astrid Meyer
    PO Box 1204
    D-27305 Bruchn-Vilson
    Germany
    glasdome@t-online.de
    (round, oval, rectangular, custom)

    Ben Bowen
    (850) 997-3797
    http://www.glassdomes.com/

The custom made domes were about 6 times as expensive as stock ones!
Good Luck