I've seen two or three people shooting .303B surplus in the last year or so and they all said they found it at a gunshow. They still paid around $1/round so not much cheaper than regular ammo. Every so often they get a FTF too.
I found a bunch of .303B IVI brass at the range around a year ago. Was told they still make a run of it every year for the Canadian Rangers up in the arctic. I guess one of them "lost" some of their issue ammo.
7.62x54R is cheap now but not much is being made anymore (in terms of military ammo that may eventually become surplus) as there is only one common weapon still in active service that uses it. It'll dry up eventually too. If you want to go that route but don't want a Mosin you could try an SVT-40. Semi auto and can be found for under $300 as a factory refurb. Of course cleaning corrosive residue out of a semi is much more time consuming than a bolt action.
Even the 7.62x39 will dry up eventually. I've heard many times that there is "too much of it to ever run out" but since it's not being made in huge quantities anymore (pretty much everyone who used it switched to 5.45x39 decades ago), and there are millions of rounds of it fired every year due to SKS' being so inexpensive, it'll dry up eventually as well. It'll just take longer since there's so much of it.
I've heard people saying; "we thought there was so much 30-06 surplus we'd never run out" and now it's long gone too.