A wide range of music no longer in copyright is available for free download online. Most of this is available as PDF scores, and is easy to open and play in PlayScore. Music composed recently and therefore subject to copyright is not generally available free.
A huge range of out-of-copyright chamber, orchestral and choral music can be found at:
IMSLP: Free Sheet Music PDF Download
Barbershop music can be found at:
Hymns are available at many sites, including these:
Free Sacred Sheet Music | 6000+ Free Arrangements
Center for Church Music – Songs and Hymns
The Open Hymnal Project – Freely Distributable Christian Hymnody for Today’s World