1. Ultimate Guitar: Ultimate Guitar offers free guitar and piano tabs for a variety of songs. You can find the piano tabs for "Fireflies" by Owl City on their website.
2. MuseScore: MuseScore is an online platform for sheet music. You can find free piano sheet music for "Fireflies" by Owl City contributed by users on the MuseScore website.
3. 8notes.com: 8notes.com provides free sheet music for a wide range of instruments, including piano. You can find the piano sheet music for "Fireflies" by Owl City on their website.
4. FreeSheetMusic.com: FreeSheetMusic.com offers free sheet music for various genres and instruments. You may find the piano sheet music for "Fireflies" by Owl City on their website.
5. Hooktheory: Hooktheory is an online music education platform that provides music theory lessons and tutorials. They also have a section with free sheet music for a variety of songs, including "Fireflies" by Owl City.