I'd be curious about that.... We buy a Chicken and Rice food packaged for a local farm supply "Del's". We've thrown away the bag so I don't know how to look for who and where it comes from.
NOT that it helps; but this is the PDF I found that says Diamond sells to private labels:
http://www.janikltd.com/article1104.pdf
Diamond also has a pretty big distribution channel to farm and feed stores too.
I can't get through on the phone and am continuing to do searches.
You could always call and ask.