Hello, building up my new road bike and having a bit of a problem trying to find a campy 9 speed rear derailleur and am wondering what else I can substitute for it? 10 speed? 8 speed? Something off the wall like SRAM? (USED stuff is perfect) For your information, I am using a campy 9 speed cassette and a campy 10 speed chorus crank (front derailleur hasn't been purchased yet).

Sorry for the stupid question, I am still new to this world.