Actually, depending on where you are the 7.62 x 54mm is starting to dry up - I used to get the 15 round brown paper packets of Hungarian MG ball all the time, but I haven't seen it in a couple years except at gun shows. Course, Winnipeg sucks for gun shops.
I do enjoy the variations of Mosin-Nagant, as I said I'm a collector and one of my main focuses is on Finnish Mosins. I have a Russian M91/30, a Romanian-issued Polish M44, an Finn M/91, M/24, M/27 and M/39. I'll post pictures later.
As far as mounting a scope on a Mosin, you can, provided you have one of the later round receiver models, it doesn't work too well with the hexagonal receivers on M91s and early M91/30s. I don't have any personal experience, because I like them original, but from what I hear your best bet is probably a no-gunsmithing mount that replaces the rear sight leaf, or a reproduction of a PU sniper mount.