I'm looking for a clean copy of the bass part for a symphony (Prokofiev's 5th) that's in copyright and isn't on IMSLP (nor is it in one of the Zimmerman books). It looks like scores are easy to buy, but I want just the bass part for personal practice. I'm happy to pay for the intellectual property, but it seems like there aren't standard sources of things like this. Am I overlooking something obvious?