Musicnotes is a good resource for finding sheet music at all levels, professional and amateur.
I have used Musicnotes for over a decade. I am a pianist and singer and I give private lessons in my home. I no longer need to purchase compilation books to get only one or two really good songs. I love my physical music library, but as time passes, all but the classical music becomes obsolete. I like being able to find what I need and purchase it on the spot. Most of the arrangements are good, and being able to transpose the music on the spot is a game changer. Musicnotes has a good range of music, including classical, as well as free things such as scales and songs in the public domain (don't quote me on that). The app has steadily improved with upgraded technology, and in the one or two times I needed help, customer service was excellent,