Andy’s Early Comics Archive
Andy’s Early Comics Archive. It has a section covering the evolution of speech balloons, and has a large number of scans of pre-cursor comic books. He only goes back to papyrus, perhaps limiting himself to non-carved sources, but there’s a good variety of stuff to keep you browsing for some time.
It’s very Eurocentric, of course. I recommend my own site for some notes on the roots of comics in Japan, for instance.