I get waterproof notesbook and regpaper books Surveying companies. I use York Survey Supply:
And there are also the Weatherwriter books which you can buy direct;
They all work - it depends how big a book you like, whether you like to take the same notes underground again, and how important a hardback surface is.
For expedition use I like the paper in the waterproof books, but mounted in an aluminium clipboard.
For personal use a ragpaper field book can work well so long as the cave is not absolutely soaking.