From Pixelmon Wiki
A Fast Ball is a Poké Ball that has a 4× catch rate on Pokémon that have a base Speed of 100 or more. It can be obtained by crafting it, buying from a shopkeeper, or as a tier 1 special drop,
A list of Pokémon that the Fast Ball gives an increased catch rate on can be found here.
Crafting
Fast Balls are crafted similarly to other Poké Balls.