Everything about pinballs

Pinballs are made out of steel. Their diameter is 1 1/16th inch (2,7 cm) and the weight is 80 gram.
Almost every pinball machine made in the last 60 years is designed for this type of regular pinball.

There were only a few exceptions:

Bally's Truck Stop (1988) used pinballs of 1 inch diameter. After designing the game it was found that pinballs sometimes got stuck on a ramp because it was too narrow. It was cheaper to sell the machine with smaller pinballs installed than to design new ramps.

Sega's Viper Night Driving had Glo-Balls. These look like regular pinballs that are covered with a layer of yellow rubber. They weigh less: 60 gram. This causes for a special effect in combination with the blacklight on the playfield. The rubber layer also makes the pinballs bounce more and react more uncontrolleable.
Glow balls didn't become a success among players and operators, especially because they felt different whilst playing, but also because the rubber became dirty very fast and easily got damaged.

The most famous and popular non-regular pinball is the Powerball in the Twilight Zone pinball machine.
This game has five regular steel pinballs but also a white ceramic powerball. Size is regular but it's lighter: only 65 gram. The powerball plays very fast. It's also not influenced by magnets below the playfield. The game has special sensor boards to be able to detect if a ball in a certain position is a steel ball or the ceramic powerball.

Sterns Avatar LE edition also uses a white powerball.

Pinball shops now also sell 'custom' pinballs. Gold or silver (be careful as it's sometimes a very shallow layer which comes off after a few games). Black pinballs also exist, even with different drawings on them.

Left to right: (dirty) powerball, Glo-ball, regular pinball and black pinball.

There isn't a factory that especially manufactures pinballs. Pinballs are just ball bearings that have certain specifications in type of steel and finishing grade.
Several types of pinballs are for sale. The main difference is their type of steel.
Regular pinballs are made out of carbon-steel. You can tumble them until they get a mirror-like finish.
Chrome-steel pinballs also exist. These pinballs are much harder (they stay shiny for a longer time, but you can't make them shine much more then they already do) but have as disadvantage they become much faster magnetic. You can order new balls at a pinball parts shop like pinballshop.nl (aff link).

Often inspect your pinballs. They should be nice and shiny, without any deep scratches, dirt or haze. During gameplay small scratches will appear and they get hazy. After a while their rough surface will start to act like sanding paper on your playfield. So often clean your pinballs and make them shiny again in a tumbler.
When the scratches become too deep you really have to replace them. How soon this is required depends on how much you play and also on the type of game: a playfield with not a lot of rubber rings and many metal ramps and ball guides can wear down a pinball very fast.

Black custom pinballs are for sale at Back Alley Creations.

Some games are real exceptions and use special balls.
Ataris' Hercules uses a real pool ball. Some other games also used special balls, but then often they're locked somewhere on the playfield and you don't play with them with the flippers at the bottom of the playfield. Capcoms Pinball Magic for example, uses a light, staged ball for the levitating pinball trick.

Ataris' Hercules uses a real pool ball.