I teach a class for TPS on integrating comics into the K-12 classroom, and I've written here before about the Yellow Kid. The Library of Congress proper is an incredible repository of early comics, but the online resources are somewhat limited. This is due, it appears, to various copyright restrictions on the material.
But if you are looking for examples of early comics (1895-1920, say) I have found a new resource in the LOC: the original newspapers. The library has recently been uploading more high resolution scans of turn-of-the-century papers, inadvertently skirting those restrictions and providing fascinating context in the process.
The comics pages were developed to boost sales, so they were often featured at the front of the paper. This makes it easy to see which papers have a comics supplement - it's the first page, and it's in color. At the time, the term comics simply denoted humorous content, so not all records matching this description contain what we now think of as comics, but if you're willing to do a little digging, there are gems. I've attached page 4 of the American Humorist from Nov 7, 1897, the Colored Comic Weekly of the New York Journal and Advertiser, which features a half-page Yellow Kid cartoon.
The cartoon features the Kid (comics' first big star - see my other posts!) celebrating (mocking?) the end of the 1897 presidential election with the other residents of his New York City tenement. It's a late effort, and a good one. The important comics historian RC Harvey wrote an updated roundup of the Yellow Kid's career in the Comics Journal last year highlighting the importance of the previously unheard-of Hearst editor Morrill Goddard. It's an excellent read and can be found here: http://www.tcj.com/outcault-goddard-the-comics-and-the-yellow-kid/
But the point of my posting is this - I found it completely by accident. Note that the record does not contain any tags or notes indicating that this is a perfect scan of a primary source featuring the most important comic character of the 19th century. I did search "comics" in loc.gov, but the search tool only recognizes standard text, not drawings or the banner advertising the "Comic Weekly". The word "comics" happens to appear elsewhere in the paper (the classifieds, ha).
Note that I'm not trying to call anyone out here. These records were clearly scanned as newspapers - that they may contain important comics or anything else of interest was clearly incidental at the time.
The takeaway here is that there are new, fascinating gems of early comics at loc.gov as full, contextual, primary sources! You just have to know where to look.