Looking for instrumental parts for musical works? These are a couple of new resources you might try. The first is IMSLP -- this website provides access to scanned copies of thousands of public-domain musical works. While the site itself isn't new, it's only in the past couple of years that they have added individual parts for large-scale works. There's an astonishing number of parts available there now!
If you don't find a part on IMSLP or in the library, you might try the new website called Partifi. It's a free and automated tool for creating parts from musical scores. The only catch is that you need to provide the score. You can upload a PDF or import one from IMSLP. After importing your score, you'll need to tag the various instrumental lines in the score. This step can be time-consuming for long works, but it's presumably quicker than cutting and pasting to create your own homemade part. After you finish the tagging you can preview the part for accuracy and then you're finished!