Beyond Selidor

caveat: I am in the UK however I'm a member of renegadepublishing which curates a massive document of suppliers of bookbinding materials worldwide. Up to you if you want to join or not (if you haven't already), but here are two paper-specific retailers I plucked from the document:

paperassist.ie - they carry Munken print paper (I use Munken for some of my bookbinding and I really like it. Munken also supply paper for offset printing to commercial printing presses). I don't know which Munken papers this store has, the Munken I've sourced here in the UK has all been long grain at A4 which means I use it for A6 quartos only.

boesner.com online store but has physical stores in many EU countries as well, not sure if they have book paper but this might be a good start to look if you haven't already