Cheapest I've found was Newegg for 1000 pieces, $46 plus shipping.
https://www.newegg.com/magnetic-ball-silver-1000pcs/p/026-09CM-00004
The alternate disk magnets are very hard to find or insanely expensive, but I found 4mm x 1mm magnets on Amazon for a better price, 250 for $10.
https://www.amazon.com/gp/product/B07DKBB3PS/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1