Interesting. I wonder if the cost of a book this old is more expensive than watercolour paper.

Etsy seems to have a fair number of sellers selling vintage pages from books.

I did see an Amazon listing for vintage 100% cotton pages but it seems to be handmade paper that is vintage looking. https://www.amazon.com/Watercolor-Vintage-Handmade-Recycled-Perfect/dp/B091CSBM64?th=1