The Nest Ball (ネストボール Nest Ball) used for catching Pokémon. The lower the level of the Pokémon you face, the higher the chances of capture. It has a green top with yellow or brown bands that wrap from the sides up around the back before ending in a circle at the top to make a woven nest.
([40-level]/10)x OR ([41-level]/10) in Black, White, Black II and White II
-Min 1, -Max 4
In B/W/BIII/WII a level 10 pokemon would have;
(41 - (level)) /10
(41 - 10) /10
3.1x catch rate
In other games a level 10 pokemon would have;
(40 - (level))
In Pokemon Emerald, you can purchasae these in Verdanturf Town.