yes APA book of Standards.
just attended the Ohio National Poultry Show! Around 8700 entries, we were there just to observe. Saw several speckled Sussex there-they are attractive. We don’t have this breed, so I can’t comment past that. However, for the breeds we do have, we found it interesting in the variability amongst a gender/breed for the ones with many entries. It was good to be able to stand in a row and look between several examples of a breed/gender to see how they varied. they had a fairly big sale area and silent auctions, some of the silent auctions were held by breeder clubs and were for breeding pairs.
You can join the APA and you’ll get their yearbook. Lots of advertisements and articles.
in my 2019 yearbook, I just found advertisements for
1. OK state Poultry Federation. Looks like shows on Dec 14, 2019 and March 21, 2020 in Shawnee, OK.
2. Heart of the Ozarks Poultry Association.
3. Canadian Valley Poultry (show was held in Norman, OK)
these are what I found closest to you in the yearbook. I was unable to find a Sussex breeders club.