Neither magnetic, nor paper: Graphite is King on the Magnetic Notes (subtitle: Paper That Clings To Anything) from the Hobonichi store.
I bought mine when I ordered my Hobonichi Techo for 2019 and paid ¥626 (~$5.50; £4.40; €4.90) plus import fees etc. This is for 100 sheets, so the price per sheet is substantially more expensive than what you might be used to from Post-its.
The back of the pack shows that the Magnetic Notes are made in Finland. You also get some advice on how to handle these ‘notes’.
You’ll find a detailed description of the ‘magnetic’ notes in my video.
Despite the name they are basically some sort of foil. Writing on them with pencil is good, but using ballpoint pens or fountain pens is, in my opinion, not that advisable.
If you want to try them out you don’t need to order them from Japan. Amazon US and Amazon UK sell them, too.
Price: September 2018
Exachange rates: December 2018