Kind of a late reply, sorry about that.
Is there a particular reason you need the score in Sibelius? With Adobe Acrobat (free) you can view pdf files, which I have found to be much more prevalent on the expanse of the internet.
The Sheet Music Archive Single Listing Page
You can only download 2 per day, but 2 are better than 0 right?
Category:Mozart, Wolfgang Amadeus - IMSLP
This one is a little newer, and has more of the orchestral and ensemble scores.
Free sheet music on 8notes.com
This one has a mix of free and purchaseable files (also pdf and sibelius) Depending on what you're looking for, they'll have more of the "pacified" pieces that aren't exactly the real thing.
Good luck and hope you find what you're looking for