Often some of the best resources are made by their modern caretakers, so I would suggest starting with museums. The Metropolitan Museum of Art's Arms and Armor Department has a number of really good guides for medieval armor, and for the study of historical military artifacts more generally. I'd look at other major institutions who are likely to have similar collections; the British Museum comes to mind.

When researching physical objects on this theme, catalogues from major exhibitions can be an amazing resource. While I'm not aware of any on the subject, give a google and see what you might turn up. For example, there is an exhibition of late medieval armor going on at the Art Institute of Chicago.

Also, for Roman/Byzantine armor, a frequent source base for studying the subject are representations in stone and mosaic. Take a look at Trajan's column or the Arch of Constantine. These are likely well troden, but will lead you to other sources.