You shouldn't really have any difficulty obtaining such mounts, but the problem is that few express rifles are sold in this country so dealers don't normally stock mounts for such rifles. The rifles are very popular in Europe and mounts are available in virtually every gunshop in France, Belgium and Germany just about off the shelf. But there again in Europe there are trained gunsmiths in most shops. The companies that Hales mention all produce suitable mounts simply do a google search and then look up their agents in this country.
Other places to try are J Roberts and Son, gunmakers of London who tend to specialise in express rifles, and also try Wilson & Wilson of Ramsgate who have sold quite a few of these rifles in the past and offer scope packages along with the rifle.
Welcome to the world of express rifles I am sure that this will just be the start of your frustrations, wait till you try to load for convergence of barrels, then the fun starts.
Incidentally what calibre is the rifle in, 9.3x74r or the pussy 8x57jrs ???