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Some of the major publishers put scores online - Boosey & Hawkes (you need to register to view them, but it's free) Music Sales Classical Edition Peters Peer Music Classical Theodore Presser - and of course there is interlibrary loan for those who aren't covered, if you're a member of a library (which you should be). Some composers who aren't affiliated with a publisher will just put up scores on their websites. Others, if you like them, you can contact and ask if they can send you anything. Composers tend to be a lot more laid-back about making such things available than are publishers (particularly if you're then going to play their piece somewhere it otherwise wouldn't have been played). Some of the really famous ones are exceptions, and can be a bit nasty, but they're uniformly overrated.