Very few customers buy directly from a mill. Generally, only the largest magazine publishers are likely to purchase directly, and they are likely to have arranged unique specifications for the manufacturing of their paper. Most paper is sold through paper merchants, who contract with one or more mills to provide their paper in specific geographic locations. Usually, several distributors serve each area, each carrying papers from different mills. So if the paper you want is not available from one merchant, try others, or ask the mill who carries their paper.
Mills are usually very protective of their merchant relationships.
Generally, they will designate only one distributor in each geographic area to carry their paper, to eliminate competition between their own merchants. A few mills will contract with some customers directly, but then often ship the paper through their local merchants. Customers get price breaks as they buy larger quantities of paper. Their primary customers are printers, retailers and major corporations. They also sell to re-sellers, who may sell paper in small quantities.