When it's core repertoire, but we've lent all our hard copies ...

... Take a look at our digital music collections.  Staff and students of the Royal Conservatoire of Scotland can download sheet music from Alexander Street Press, or from LibrarySource.com.  They're listed on the Whittaker Library electronic resources page, and very usefully augment the popular free resources that anyone can access. 

Partifi.org
Catherine also found an intriguing website called Partifi, the other day.  It can create parts from an IMSLP (Petrucci Music Library) full score.  It's fair to say that it's a bit fiddly to use, but if you're desperate to create parts for a piece of music, and they're not already online, then you might find it handy.

It's listed on our electronic resources page, to help you find it when you need it.