This is not intended to be an exhaustive list, but I will keep records here of places with FP-friendly notebooks and paper supplies.
Ships from Australia.
Stocks notebooks and looseleaf in Sanzen TRP, Iroful and a wide range of other papers.
https://www.etsy.com/au/shop/TarokoShop
Currently on hiatus, stocks notebooks in several popular papers (e.g. Sanzen, Iroful, etc) - worth checking when they come back online.
Note: may be permanently closed?
A small range of well made notebooks using Japanese papers.
Their flagship Crossfield notebook is 480pp Sanzen TRP (their current 8th Edition uses Sanzen paper from before July 2024, i.e. likely escaping the ‘bad batch’ issues.)