Another source for display cases is to keep your eyes open for stores going out of business.They somtimes sell off most of their display cases. I have a great case about 7 foot long 12 inches deep and 5 foot high. Now if i can just get all the wifes stuff out of it I would be set.
If you have an IKEA near you they have some very nice cases
IKEA | home | Bookcases & storage | Bookcases & cabinets | Glass-door cabinets
Also check out their Billy line of book cases. You can get glass shelves and doors for them
IKEA | home | Bookcases & storage | Bookcases & cabinets | Bookcases | BILLY storage system