It sounds like what you have is a Model 1895 Austrian Mannlicher Straight-pull rifle. IF it IS in the original 8x50R [Austrian, NOT 8x50R Lebel] chambering, you've got a rare duck as most, if not all, were re-chambered to 8x56R, so before going any further, you should do a chamber cast to determine what it is actually chambered for, as some were also converted to 7.92x57IS [aka 8x57 Mauser].
Ammunition has not been made in many, many years. AFAIK, the last ammunition made was Kynoch ammunition, which will be marked .500/320, but supposedly ammunition was made in India for the Lee Enfield rifles chambered for the cartridge, which is known there as the .315 Indian.
I doubt you will find ammunition for the old girl & if you do it will be both expensive and, quite likely, corrosive.
If you already have dies, try to form a case from 7.62x54R. If the rim is too thick +/or too large in diameter to close your bolt, you'll have to trim the rims to size, then you'll have to resize & trim. You can also resize & trim 8x56R brass.
If you do not have dies, try CH-4D.
Let me guess, the vendor told you that ammunition would be easy to find, right?