Nicolle, David. The Age of Charlemagne. Warfare in Western Europe 750-1000 AD. Osprey Men at Arms series, 1984
The main image on this page comes from a close up of statue of St George, Nikolaiviertel, Berlin.
The Wikipedia page on Byzantine costume is good and gives you an idea of what ceremonial Monsalvat wear would have been.
La Fenici Historical Archive – images from a production of Parsifal in 1983. Peter Hofmann was never knowingly overdressed.
The manuscript images of Parzival on this site come from Universitätsbibliothek Heidelberg, Cod. Pal. germ. 339. Wolfram von Eschenbach, Parzival (Band 2): manuscript — Hagenau – Werkstatt Diebold Lauber, um 1443-1446.