My collection of free sheetmusic includes scans of several old mandolin methods and other collections including methods by Cristofaro, Cottin, Pietrapertosa, Fletcher, Vorpahl:
The best links to free sheetmusic sites for mandolin and guitar can be found on this page:
The Petrucci Library (IMSLP) is the largest library of free sheet music. You can find mandolin methods, music for mandolin solo and mandolin in combination with other instruments. Use the following buttons to get directly to the selections of mandolin music:
I am publishing mandolin sheet music on the musicaneo site. You can find many free editions for mandolin and other instruments as well as some scores that you have to pay for.
On my German website mandoweb.de you can find some more links for sheet music for mandolin: