Here are some references that can be downloaded for free:
The American Poulterer's Companion. a Practical Treatise on the Breeding ... By C. N. Bement (info. on JS's stock)
Inheritance In Poultry C.B. Davenport (brief quote from a letter from Sebright's son telling a slightly different story)
His 1809 pamphlet: The Art of Improving the Breeds of Domestic Animals in a letter addressed to the right hon. Sir Joseph Banks, K.B. by Sir John Saunders Sebright, bart., M.P.
Available in full (reprinted) in: The American Farmer 1825, John S. Skinner
The paragraphs from that pamphlet that indirectly contributed to much hullabaloo:
`Natural' as compared to `directed' (human intervention) `selection'. Google Herbert Spencer.
Google Books/Internet Archive/Project Gutenberg (some places to run down the above - and more).
Have fun.
P.S. if using windows, and either Firefox or IE, Adobe Acrobat reader replacement (the free version is about as good as the $), not a resource hog, faster and has more features (
http://www.docu-track.com/product/pdf-xchange-viewer ) whacking up/resorting/cataloging reference material from PDF's in a hurry. Notepad ++ is good for quick work in plain text files:
http://sourceforge.net/projects/notepad-plus/ (also free)
(Mrs. Beeton's Book Of Household Management notes the location of `Sir John's' coops... )