The Göss Vestments have many interesting embroidery motifs. I used a couple of them to embroider a pin cushion.
Manuscript marginalia are awesome, but I'm not sure about the copyright issues since you want to sell the patterns. Maybe medieval tiles* are less covered by copyright - at least if you photograph them yourself?
*: for inspiration: http://home.nordnet.fr/ccogo/LECARREAUMEDIEVAL/ENGLISH/catalogue.htm
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