I’m wary of recommending this because the site has a confusing interface but it is the only source I know of for professionally marked parts (and conductor scores).
New York Philharmonic | Digital Archives
You can find marked scores and parts used in performance by Masur, Bernstein, Kostelanetz, and others and by the principal section leaders of the times.
You just have to play with it to get a handle on the navigation. For instance try entering in the search field “beethoven symphony 9” (forget the dates for now).
You’ll get to a page with way too many hits. But look on the right side and you’ll see filters for programs, scores, parts.... If you press “parts” you’ll get a pulldown that lets you select instrument. Or you can put the instrument in your original search but then you won’t be presented with the conductor scores. They’ve changed the interface since I last used it so I’m still exploring it myself and sorry I can’t give you a better introduction. One good thing they’ve done is to ditch the copy protection they used to have: translucent treble and bass clefs plastered over the part to prevent you making a screen capture. Good riddance.
Thanks for the newer resources above. Lovely.
For some reason I couldn’t get the excerpts to play in the Double Bass Excerpts (LouisF). Kept telling me I was in the wrong country.
An older site I do know where sound files work (and you can adjust their speed) is
Orchestra Excerpts - Study Audition Excerpts and Win
Just scroll down and push the button with the bass fiddle and you’re on your way. The online use is free.
PS Addendum, just exploring NY Phil site some more. Holy cow, they’ve got Mahler 1 & 4 marked by Mahler!
