Something I've always wanted to find a source for are composer's skethes (preferably something readable).
As an example the Library of Congress has some of Copland's sketches available online. I like looking at them while I'm listening to the pieces because they are boiled down to their essense without wading through all the orchestration and I can see voice leading, etc. very clearly.
It would also be nice to see the progression of ideas and development of a piece of music to sort of "get inside" a composer's head. I don't know if there are any other such sources, but maybe someone else knows.
I agree with you. It is difficult to find sketches from most of the composers, mostly (I think...) because of copyrights and intellectual properties... This means that most of the sketches will end up in a dark room of some national library or museum as historical documents after the composers' death, and will only be available for academical work, not for general public.
On one hand, I can understand, being myself a composer (only amateur, but a composer, nonetheless...), that one must keep for himself the secrets of his work, especialy in music, where's easy to find many 'copycats', even from historical composers... On the other hand, this really difficults the task of those who simply wish to study music indepth, in order to achieve a better performance as composers or musicians.
So, bottom line is that on-line resources like this aren't easy to find, unless one knows exactly what he's looking for. And the ones which are easier to find, will be historical composers, as their manuscripts aren't covered with so many copyright rules...
Yes, this is the 'Digital Mozart Edition‘ where you can download every page of Mozart's oeuvre for free. However, it's not so easy to find all the sketches and drafts by Mozart of which there are hundreds – they are spread all-over the 127 volumes of the NMA! But if you go for the individual genre series and look for a particular volume you'll find a lot of sketches, drafts and fragments in the 'Anhang' of each volume – quite ready for study! Note however: All text is, necessarily, in German – but, happily, the language of musical notes is universal.
Let's take an example. Click on SERIES IX: Piano Music, hit the red i symbol (= Table of Contents) of # 98 (= Piano Sonatas vol. 2, NMA IX/25/2 …) and click on the newly opening page down under ANHANG at the PDF symbol of "Sonantensatz in C KV6: deest". You'll get a wonderful sketch of 25 measures of a sonata movement which you'll be able to easily play on your piano and, if you like, to properly continue with some improvisation.
Have fun with studying Mozart sketches in this way!