And the website:
http://www.magna-matic.com/blade-balancers.html
Another brand:
https://www.discountonlineparts.com/lawnmower/?c=Tools&sc=Blade%20Balancers&b=&id=9689
Popular model:
https://images.duckduckgo.com/iu/?u=http%3A%2F%2Flawnchat.com%2Fwp-content%2F...