This is a Dead Thread Walking, anciently old, a new record, and now reanimated to Zombie Thread Walking; It is a common newbie mistake to see and post on one; You see recommended threads by The Algorithm (all hail our Algorithm overlords), without noticing the posting date. No worries, it's something we all learn.

I will answer your question because it is easy. If the bottom bracket is old style cup and cone, meaning loose ball bearings, replacement balls are easy and cheap to find at bike shops or online. If they are of an unusual size, a visit to an industrial hardware or bearing supply company, and bring balls, spindle, and cups, and I'm sure they can find something that works. In Detroit it was Detroit Ball Bearing. In Seattle it's Tacoma Screw. However the cups (outer races) and spindle (inner races) are not found at bearing suppliers, so if damaged, you need to check with a bike shop that handles older designs. If no replacements available, if you are handy and have tools, you might be able to restore the spindle by mounting in a lathe and fixing a small rotary grinder like a Dremmel tool, or air grinder, to the carriage, and carefully re-grind the surface as the spindle rotates, sort of a poor man's jig grinder. Same for the cups. I've done crazier things to fix something.

If the bottom bracket is threaded to a standard bike size, you can replace with a bottom bracket "cartridge" bearing, if it will fit the crank.

If the bike has individual sealed bearings at each side and pressed in, replacements are also at bearing supply companies as noted above, they can measure, or more likely, view the tiny numbers on the side of the bearing and cross-reference to a new replacement bearing, however you will probably need a bike shop to replace the bearings.

Many solutions are possible, but it starts with a bicycle mechanic/technician with excellent knowledge of all bottom bracket types.

Welcome to the forum.