Master Ball is the best one of Pokeballs, which is used to capture a wild Pokemon 100 percent. However, Master Ball cannot be used now in Pokemon Go. The image of Master Ball is founded, but there is no official announcement.
Master Ball does not appear as a reward for level up. So, that is not correct.
Master Ball is expected to be available to catch Mewtwo at the official event.
Currently there is Pokeball at the in-app shop. So, Master Ball is possibly become purchasable in the future.
