Amazon.comにも出品されているようだけど、購入はAliExpressでよさそう。

Kase 77mm Magnetic Step-Up Adapter Ring Kit ( Convert Thread Filter to Magnetic Filter)

https://www.aliexpress.com/item/1005001590360894.html?spm=a2g0s.8937460.0.0.73cc2e0egR9mB3

Kase 77mm / 82mm Magnetic Adapter Ring ( Convert Thread Filter to Magnetic Filter )

https://www.aliexpress.com/item/1005002857245210.html?spm=a2g0s.8937460.0.0.73cc2e0egR9mB3