If you're looking for sheet music already scanned, then
SheetMusicPlus.com has what you're looking for, though it's not free. It's not a program, but it'll find you the music.
The problem you'll be facing is online/international copyright laws that prevents music that isn't Public Domain (composer has been dead for at least 75 years OR the composer willingly allows the music to be free access) from being freely accessible to the public. The only free music I've been able to find with consistency are church arrangements, opera scores (FULL opera scores for free online

), struggling artists and some guitar music here and there.
If you're looking for a music-scanning program, there is no better program than SmartScore. In addition to scanning music and doing what you need on THAT end, if you happen to have the Finale music writing program, you can scan music with Smart Score and it will be automatically transferred into an editable Finale File. Smart Score used to have a free trial/demo version, but in my brief searching, I have not been able to find it.
I hope I have helped!
Ransac, cpa trash man