Try
www.ssfirearms.com if it's not in their catalog, give them a call, often they may have such things in shop even if not listed. www.cooncreekoldwest.com as well. You will have to call the folks at Coon Creek, often if they don't have it, he knows where to find it.
In fact, the artillery helmet plate is listed in the S&S catalog on page 95, Item # IW45, $50. I ordered on of these from them last year, was on my Helmet at the SASS convention. This is the M1881 style. On page 93 is the M1872 Eagle plate, #IW6, which has no branch designation.
The S&S catalog is a great reference book to have around.