I've recently discovered what must be one of the best bookdealers for secondhand, old and rare fishing books in the world.
Laura Hiscock at Chalmers Hallam Books in Hampshire England has thousands of books on game, course and sea fishing. She produces catalogues several times each year usually with 1500-2000 titles.
Prices range from UK£10-UK£5000. All the books are realistically priced and accurately described.

I have purchased many outstanding volumes recently.
Give them a go.

The website is www.hallam-books.co.uk