You may have seen that the British Library posted over a million public domain images online recently. I've been poking around there. The images that struck me right off the bat were the detailed portrait illustrations. I thought they might work well as npc portraits.
I spent about an hour working on a single page compilation of some of these, but had to make the faces so small they lost a lot of detail. I decided you might find them more useful as individual files. Anyway, these are the ones I considered most interesting from the first 33 pages of images: