No Sense of Place points to a great collection of early 20th century animated films at the Library of Congress. Includes Krazy Kat and The Katzenjammer Kids, among others. But no Mickey.
The movies section at Internet Archive also has some great stuff, but it takes a little bit of searching to find the good ones (look for John Hubley).
And finally, when you’re tired of squinting at small QuickTime movies, stop by Oscars in Animation: 7 Decades of Animation Art if you live in the LA area. Can’t beat looking at the real thing.