On a re-read I noticed this during the scouring.

Sam hurried to the house. By the large round door at the top of the steps from the wide yard stood Mrs. Cotton and Rosie, and Nibs in front of them grasping a hay-fork.

'It's me!' shouted Sam as he trotted up. 'Sam Gamgee! So don't try prodding me, Nibs. Anyway, I've a mail-shirt on me.'

Where did Sam get the mail shirt from.