Army painter do a box
The Army Painter | Paint Mixing Balls -| Rust-proof Stainless Steel Balls for Mixing Model Paints - Stainless Steel Mixing Agitator Balls, 5.5mm/apr. 0.22”, 100 Pcs https://smile.amazon.co.uk/dp/B07Q23FVYR/ref=cm_sw_r_apan_glt_fabc_8Z4R5YA25CHK0G3BHGWF?_encoding=UTF8&psc=1
https://www.amazon.com/Army-Painter-Paint-Mixing-Balls/dp/B07Q23FVYR
Metallics can be a pain in the ass, you gotta really stir up that mica.
Edit: If you want to step it up a notch decant your bottles first.
Hey again bud (me again from earlier).
Something people often do with dropper bottles, is buy a small pack of ball bearings, much like these: https://www.amazon.co.uk/gp/product/B07Q23FVYR/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
And you can place them inside the dropper bottles. It makes it a lot easier shake the paint before use. I know it's yet another expense, but it's not too much and they never break or run out. Once the paint pot is finished, you can simply take it out the pot and put it into the new one.