Bonjour Phil
As Don mentioned it, Egun is always a good source for finding such items (there are currently two Khales Mignon for sale, unfortunately not with Viennese mounts); I would also check regularly auctions from Holt's, Springer, Hermann Historica, Dorotheum and the like. Another option would be to buy a low cost MS mounted with Viennese mounts and a period scope at an auction and transfer mounts and scope to your own rifle; some years ago I purchased such a rifle at Holt's, but as at they time I did not want Viennese mounts they were gifted to an American Forum member. Last, if you are prepared to swallow the cost, you may buy the scope currently for sale on Ebay and have it legally exported from the US by a registered exporter; ITAR doesn't prevent exporting such items providing one is prepared to enrol in burdensome admin and associated paperwork, which an export company would deal with on your behalf.
Bonne chance.
Louis