From Pixelmon Wiki
A Nest Ball is a somewhat different Poké Ball that works especially well on weaker Pokémon in the wild. The Nest Ball's catch rate is based on the formula: 41-Pokémon's level10×, with a minimum catch rate of 1×. It can be obtained by crafting it, buying from a shopkeeper, or as a tier 1 special drop,
Crafting
Nest Balls are crafted similarly to other Poké Balls.