Try Speed Graphic. Lots of choice, bit more expensive than ebay but you get a wider choice
http://www.speedgraphic.co.uk/cat.asp?c=174&1=Umbrellas
There's a wider choice in one shop than on all of ebay worldwide?
That's what I thought, but I tried entering Photographic Umbrella into ebay and only got 2 hits. I suppose it depends on when and where you do the search.