Have you tried Hendrix Equipment Supply? It is retail sales as well. They don't have a cloche by name, but they have these which are close in design:
http://www.hendrixequip.com/p-281-oatmeal-dish.aspx
http://www.hendrixequip.com/p-283-oatmeal-dish-lid.aspx
Might also try Crown Food Equipment:
http://www.crownfoodequipment.com/ I've found some obscure kitchen gadgets in there before.
Worst case, what about these? Metal Plate Cover (unfortunately it's a set, but it would give you a similar look as a cloche):
http://www.bedbathandbeyond.ca/store...d=serving+dish