The Master Ball (マスターボール Master Ball) is the best and rarest ball available in any game. It has a two-shade purple top forming two brighter purple circles on the sides of the top, as well as the letter "M" printed on the top. The Master Ball has a 100% chance to catch any Pokémon, no matter what the circumstances and could be described best as "the ball that never fails".
It was originally manufactured by Silph Co. in Saffron City, but only a few were produced before Team Rocket tried to steal them. Since then, they have been discontinued. From then on, the few remaining prototypes were only given to accomplished Pokémon researchers.
Only one can be found during the course of the story in each game. However, starting from Generation II, additional Master Balls can be won, albeit very rarely, from the in-game Lottery. More can also be gained by trading a Pokémon holding a Master Ball from another game. It is possible to get 450 master balls without hacking.
The Generation V games are so far the only ones where it is possible to get more than 1 Master Ball without winning it from an in-game Lottery corner.
Though production of it is believed to have been discontinued by Silph Co following Giovanni's and Team Rocket's attempted takeover, given its appearance in other games it is more likely that the company still produces a limited number of Master Balls for use mainly by Pokemon researchers (like Prof. Oak and Prof. Elm), high ranking members of the Pokemon League (such as the Elite Four and League Champion), those given away as Lottery Prizes, and possibly some for use by Silph employees. It is also possible and likely that due to their capture rate their production and distribution is monitored and regulated by Pokemon League. Given Cyrus having a Master Ball in his possession it is likely that a few may have made their way into the hands of criminals via either theft or purchased on the black market.
The Master Ball's rarity in both the games and anime is likely due to several factors:
Team Rocket's attempted takeover of Silph Co and attempt to steal the Master Ball (in the games only).
In the both the games and anime, its high capture rate would make catching wild Pokémon too easy and if it where easily available it could cause the number of certain pokemon in the wild to decrease due to over catching.
If Master Balls were commonplace, it would likely render all other models obsolete, which could force Poké Ball manufacturers to discontinue some and/or all other types of Poké Balls, and its high capture rate would reduce the number of Poké Balls purchased by Trainers, due to a lack of failed captures (in the games, if a capture attempt fails the Poké Ball used is lost, a fact which encourages Trainers to purchase more Poké Balls to prevent running out and/or purchase Poké Balls to replace the ones that where lost due to failed captures).