Having spent many hours reading about so many personal reports of the growing scarcity of most calibers of surplus ammo, it is my impression that low-cost (corrosive) x54R ammo will remain widely available for a while: Bulgarian, Czech, Polish, Hung., Russian.
Some private sellers offer 8 mm from .25-.30/round, and some firms such as AIM or JG sell it. A good search is "Ammo-deals.com".
The reports are that surplus 8 mm will become as scarce as surplus .303 in a few years, if not sooner.
It's kind of funny...I walk into small town gun shops and often when asking whether they know who might have Low-Cost Surplus .303, they steer me towards over-priced new Remington.
Apparently some people don't understand what surplus is, or hope that we are too desperate or a Dummkopf who doesn't know the difference.