It’s available.
7.62x54R - normally comes in 440 round tins for surplus.
Avoid Bulgarian - Any Russian, Polish, Albanian, etc. are ok. The ammo is corrosive...no biggie, just shoot windex down the barrel and around the bolt face (actually you can even use water) when done shooting to flush the corrosive salts. Then clean as normal.
May also want to buy a shoulder pad unless you’re wearing a large Russian Cossack jacket :-)
If you ever get a batch of very old Russian ammo that kicks so hard it causes the bolt to jam, discard it. The Soviets had a WWII era vehicle mounted machine gun that used the same ammo as the Mosin-Nagant, but they loaded it a lot hotter than the rifle ammo.
There used to be a webpage with photos of the head stamps that you had to be concerned about.
I have crates of 1953 head stamped Bulgarian.
It all goes bang and it hits what you point it at.
Mildly corrosive, swab the bore out with windex