The surplus we buy now was made and stockpiled for WW3. The USSR was getting ready in a big way for a huge conflict and manufactured billions of rounds. Their production now is a pittance by comparison with very little being sold as surplus relative to the Cold War stockpiles. Saying that they still use 7.62x54R so we can still get surplus until they stop using it is like saying people still use steam locomotives some places so there should be plenty of run down engines to be sold as scrap. Just because something is still in use doesn't mean it's still being mass produced at the levels that result in huge amounts of surplus. It wont disappear but prices will steadily rise.
The same goes for 7.62x25 and 7.62x39. Both are still in use but not nearly as much as in their golden years. Almost all countries that used them have switched to 9x18, 9x19, or 9x21 for almost all of their pistols and 5.45x39 for all their main infantry weapons. The African warlords, terrorist insurgents, and various freedom fighter groups around the world using AK-47's are largely tapping into and using up ex-Soviet surplus ammo reserves, just like us Canadians. I remember seeing one picture of ISIS fighters with crates of 7.62x39 and realized the crates had identical markings to the crate sitting in my garage. It wont truly run out because it is still being made but once the huge glut from the Cold War is gone prices will increase, probably significantly. Even the new manufacture Norinco stuff is only as cheap as it is because it's still somewhat competing with the Soviet surplus. Once competition is reduced, prices will increase.
As the saying goes; 'buy it cheap and stack it deep'. A lifetimes supply now may cost less than a 5 year supply later.
