Roman Shields
You need to get your hands on a facsimile edition of the Notitia Dignitatum, a late Roman manual on the post-Diocletian provinces, listing titles and, amazingly enough, displaying the roman symbols of different provinces and offices.
The book was enough of a hit in the middle ages to survive, and the images used for coats of arms were recopied many times. Of course, the images in the book are medieval renderings of late-roman images, and because the monks had a very different visual sense and a different way of thinking about form/line/color, what you see is not what went into the original editions. There is, however, enough graphic information to reconstruct something that would be very close to the original, if you can get the late classical style right.
Another thing you should take a look at are the marginal/incidental illustrations in the Peutinger table. They show the way Romans symbolized towns and cities when they put them on a map.
