Music International Music Score Library Project (IMSLP) It's a great resource if you're away from your sheet music and need your score quickly to practice with. Granted, it might not be the exact arrangement you've bought already but it'll be close! As of 19 November 2010 it holds 78,000 scores. You can see for example - Elgar's Salut D'amour - piano solo version, piano and violin, piano and clarinet, and even the orchestral version too!
I recently performed the piece. I couldn't wait to get started so I used the free version from the website to practice with until I bought the Peters edition version..
Read about the International Music Score Library Project's history on wikipedia