I have 5 or 6 mosin's(mo-seen). One of them is a Finish M39 (A mosin captured by the Finnish and upgraded). Love those cannons. Be careful, they'll shoot through some thick wood. PM me (cause I'll forget I was on this thread) and I'll share some links on the mosin and ammo. Lately I've been able to buy a spam can (440 count) 7.62x54R and have it shipped to the house for $100-$120. Some gun shows will have the ammo cheap as well - less than $100 or 3x's the price. I usually look for the Alabama Ammo group, they will most likely be the least expensive.
Two things about the ammo you need to know. Number one is salt. The spam can is surplus Russian ammo. I've heard a couple of different takes to clean it. Remove wooden ware after firing and run hot water through it. And/or windex with ammonia.
The other thing is where the ammo came from. I can take two mosins shoot Russian ammo, then shoot Bulgarian ammo and one will shoot one higher than the other. Get two different rifles and the ammo that shot low in one will now be shooting high. If it shoots high fix the bayonet to it. That will usually bring it down a couple of inches at a hundred yards. Some folks will put something on the front sight to make it taller.