Ammo for most is not a problem, but most now (except for the 54 Russian) is factory and not surplus. Even most of the 8mm is gone.
The K-31 is an awsome rifle..why anyone would pay $200 for a Mosin (and I own four of them) when you can still find a K-31 for about $300 is beyond me.
The K-31 is head and shoulders above even many modern rifles in terms of quality, and accuracy. Interestingly enough, the GP-11 surplus, while not cheap, is AMAZINGLY accurate for surplus ammo. I've been unable to handload anything that performs better- and that's VERY odd for factory ammo of any kind, let alone surplus.
Not cheap at $.50/round- but you didn't say "cheap"- just available. It is now, but seems to be getting scarce.