Pool balls are currently a useless item, and often considered as junk.
Pool balls have no use, being added purely for the purpose of clutter, creating more realism for the player when looting.
Pool balls can be found in bars.
From newitems.txt (Project Zomboid directory/media/scripts/)
Weight = 0.2,
Type = Normal,
DisplayName = Pool Ball,
Icon = PoolBall,