AMMO by MIG Accessories - Tools for Model Building
Item: Stainless Steel Paint Mixers

From the Manufacturer:

This set includes between 70 and 80 stainless steel balls (5mm diameter) perfect for use as agitators in any paint. Add a pair to your jars and paintwork will be much easier, faster, and effective. To make a paint reach its optimum performance, it is necessary that all components are thoroughly mixed. Valid for acrylics, lacquers, enamels and any other type of liquid paint. Will not rust or alter the colour.