IMSLP stands for International Music Score Library Project. The website is a great resource for finding sheet music which is no longer protected by copyright. Looking for something that is obscure or out of print? Try IMSLP.org!
If you’re looking for sheet music, I recommend Sheet Music Plus. They have the world’s largest selection, guaranteed low prices, and great customer service. Plus, if you follow links from my website, a portion of your purchase price will be returned to me to help me maintain my teaching studio!