There are no readily accessible sources that you're overlooking, and, yes, there ARE some Keystones which only exist in inferior quality.
The problem with the Keystones is that they're also frequently incomplete - thus, you'll need to find several different versions of the same film to have all the available material.
Some instances of this are 'Kid's Auto Race' - where the Blackhawk print is missing the last minute or so (same with 'His Musical Career'

. Various edits exist of 'Mabel At The Wheel' and 'The Fatal Mallet.' Until recently, I'd never seen a version of 'Mabels' Busy Day' where the heads weren't cut off, and the same probelm afflicts 'His New Profession.' Different sources exist for 'Laughing Gas,' 'Property Man,' 'Face on the Barroom Floor' and 'The Masquerader' as well. There are differences between prints of 'Dough and Dynamite' and 'His Trysting Place' and, finally, a number of Keystones have even been edited down to one reel, such as 'Caught in a Cabaret,' 'The Property Man' and 'His Prehistoric Past.'
And, of course, 'Tillie' exists in seemingly endless variations, all of which are incomplete.
Someday, when I have a DVD burner and telecine in hand, I'll begin putting the pieces back together (for my own benefit), simply because I don't see anyone else doing it (well, David Shepard's been talking about it for years, but talk is cheap, right, Mr. Totheroh?

).