I also bought my Webley holster from eBay. There is a great eBayer from UK, who sells original Webley holsters and other British Militaria from WWI. But he doesn´t offer guns or propguns.
Also...if you make a trip to the Cairo Bazzar it looks like G-Mann has just put one up for sale. You might contact him if you havnt figured out where you are planning on purchasing a holster at just yet.
The revolver used in Last Crusade and Crystal Skull is a Webley WG model. As far as I know there are no reproductions available of the Webley WG. There are repros of the Webley MKVI but it is not the same as the WG model.
As far as the holster is concerned, yes any of the Webley holsters pointed out by everyone else is very close to the one in the Crystal Skull photo. The major difference being the "plug" found on the those Webley holsters. The one Indy is wearing in the photo does not have a "plug".
If anyone can find a source for an "unplugged" holster that would be great!