A lot of free font sites contain fonts that might not be appropriate for printing (due to license/copyright issues) which shouldn't be a problem for a web-only comic, but if your thing was ever going to be published on paper it's wise to pick up a font that you're sure is ok to be used in print.
Thus I suggest two sites.
http://www.larabiefonts.com/ - Larabie fonts: 384 fonts for personal and commercial use.
http://moorstation.org/typoasis/designers/klein/ [moorstation.org] Manfred Klein Fonteria: not sure how many but there's probably couple of thousands of them. Let me quote from the website:
Manfred's fonts are free for private and charity use. They are even free for commercial use - but if there's any profit, pls make a donation to organizations like Doctors Without Borders.
There you go, my tiny contribution :D