Paper merchants

These people normally sell papers in large quantities, so check what the minimum order would be.
You can ask for free samples (you can tell them you are a student or a freelancer...)

GF Smith

Fenner Paper

Fedrigoni

Paperback

Bookbindng tools & materials

London Centre for Book Arts (Hackney Wick)

Shepherds (Victoria)

Ratchford (online)

General art supplies

PDA Card & Craft (online) You can buy papers from 1 sheet!

Paperchase (Tottenham Court Road)

London Graphic Centre

Risograph, newspaper printing

Ditto Press

Hato Press

Newspaper Club

Professional bookbinder

Wyvern Bindery (Clerkenwell)