We have developed a product that we think is ideal for infomercials. 
We think it best to partner with someone like Guthy- Renker, who can
"take it from here" in terms of developing and marketing the product
in this channel.  So, what I am looking for is a list of other
companies, like Guthy- Renker, who are major players in the
infomercial world, who market their own products.  I am not looking
for companies that will produce or otherwise assist in developing an
infomercial for US.  Hopefully the distinction is clear, but if not
let me know...

I would connect with some of the infomercial companies in the industry
and ask them directly.  You might start with Hawthorne in Iowa and go
from there.
Response Magazine is available online and might have a directory of
infomercial producers.