One thing you can do is search images at the Library of Congress. They often have archival scans of works available, and one can use them for all kinds of projects.
It may be of some interest to you that I am a collector of British Billheads only (I live in England). There appears to be very little interchange between the 2 groups and therefore (bearing in mind the engraving process) this suggests that all the images I hold would be new to you. It occurs to me that there may be noteworthy differences in the engravers technique, subject matter and style.
In terms of heavily illustrated billheads, the English market sees relatively few Victorian (1837 - 1901). There are far more from the period 1900 to the Great War. Many firms continued with the tradition up to the 2nd World War. After this some firms continued with illustrations but used more modern graphic and printing methods. British billheads of this era will also carry a revenue stamp (stuck on or embossed) if the transaction has been validated.
I would be pleased to supply some examples to investigate my theory. It would be simple to mail you a few scans but I have only a standard commercial scanner (Max scanning colour depth: 48-bit. Max B&W scanning resolution 1200 x 600 dpi) which.I suspect is too low for your needs.
In any event I would be pleased to hear your views and see some of your work. Do you have a website?
Hop over to my primary website http://toolemera.com and look at the Manufacturers and Merchants section. There are numerous billheads and letterheads scattered throughout that section. If you come across something of interest, I'ld be happy to make copies of the original scans for you. All are scanned in TIFF, most in color, at 300 dpi or greater for the master copies.,