They are metal 15mm or 0.59 inch balls, found in long cartridges containing up to 15 balls. These can be bought in most shops in the game and have the best exchange rate as 1 MGR buys 8 pellets, though selling back is practically pointless, given how many you need to sell to get even one MGR. Because the ball bearings is the cheapest ammunition available, you will not gain much by selling these back; only trade MGR for Ball Bearings if you are using Tihar or Hellbreath a lot.
These are rather uncommon to stumble across in-game, and are not as common as shotgun shells or dirty rounds. While on lower difficulty in Metro 2033, each pickup will gain 15 ball bearings to the player, on the highest difficultly the player will only get 5 to 8. Furthermore, ball bearings are also the cheapest form of ammunition, as one military-grade round will net the player eight ball bearings at an exchange kiosk, so buying most of your ball bearing ammo is a viable strategy, especially since the guns that use ball bearings are high-damage weapons, that can kill anything in one shot with a headshot (as long as the weapon in pumped/charged enough). Given their rarity, if they are regularly used by the player, they should buy as many as they can, whenever they can.
These are only used in the Tihar and Volt Driver in Metro 2033, so it it best to carry a backup Kalash or Uboinik in case you run out of pellets. If you are using either gun as your primary, you should buy more bearings whenever you can.
They see even less use in Metro Last Light, as the main campaign only has the Tikhar (Tihar), so their use is diminished until you can find one. The first Tikhar available to you is found in the Facility level, on the walkway above the gas room, with some rounds. Ball bearings are uncommon in Last Light, so you should buy as many as you can when you reach a station - you can carry up to 90 bearings. They are also used in Last Light's DLCs, as ammunition for the Hellbreath (Volt Driver).
Metro: Last Light: