This could be old news to you, but maybe not! I recently discovered that Wikimedia Commons has a specific category for illuminated manuscripts by century, which includes TONS of gorgeous manuscript illustrations from the 2nd through the 16th century — and these illustrations are a fabulous source for medieval costuming.
[Edited to add: Might have been nice if I’d added the link to illuminated manuscripts by century in Wikimedia Commons!]
In particular, I am stunned by the gorgeousness of De Mulieribus Claris, which is from the late 15th or early 16th century. Here’s a small sampling of images: