The French National Library has a great mass of medieval manuscripts online at its Gallica site. Finding them, however, can be very tricky.
Some time back, a genius drew a chart of how to do this. It does work – my rather Covid-addled memory tells me only that I did work through it. I probably wrote something about it here, but I can no longer locate it.
Today I found a copy of the picture on my desktop. Unfortunately I don’t know who drew this (if you do, please shout!) and I thought I’d pass it on. Magic.
Update: Nov. 2022. The step 2 doesn’t work but the department de manuscrits page is here.