The British Library Catalogue of Illuminated Manuscripts
As far as I know, this hasn't been posted yet: a searchable database of scanned manuscripts from the British Library.
Catalogue of Illuminated Manuscripts
For example, using "map" as a keyword will turn up 53 hits. Half aren't actually maps, but the results include some beautiful, and very old, hand-drawn maps. The scans are large and high-quality. Some include the color reference bar!
It's also a nice reference for decorated initials, calligraphy, and such.