Have you checked Royal steel Ball, $1.50/lb any size.

 

Based on the fact that pachinko balls do not need to be made to such exacting dimensions as ball bearings, you would assume that they are a bit cheaper.

Royal Steel Ball as mentioned by Treefork above however, sell all their non perfect balls as slingshot ammo at a considerably reduced price.

 

umm... they are ball bearings. nathan sells them for 3.50/lb shipped, those are $30 for 300.