This is a great website that provides free sheet music for various instruments as well as music theory lessons. For private piano lessons, I search for classical and popular sheet music. There is sheet music for levels ranging from 1 (easy) to 5 (most advanced). The score arrangements appear to be suitable for beginners. The arrangements also make it an easy transition for when students are ready to learn to original scores.
This wiki page categorizes sacred music into holidays and holy days. It not only shows the piano and choral sheet music in a pdf format but also gives a description to each piece and why it is appropriate for a specific holy day.