About 18 months ago I had to decide on 54R or 8mm. Cost & availabilty was about the same for each... then. Not no more.
Some say 7mm Mauser, .303 Brit, 7.62 Argentine, 6.5x55 and 8mm et al, will come back. I hope so. Dought it, but hope so.
The Mosin action isn't a Mauser, but the ammo is still around and at ~.22cent to .25cent a round depending if you prefer Tuna or Spam Can varity.