iTunes 10.5 first beta was released on the day of its announcement along with iOS 5 and Mac OS X 10.7 Lion at the WWDC event in June. And after the first beta was handed over to the developers, 3 more beta releases of iTunes 10.5 came in along with the iOS 5 test builds.
The next beta build of iTunes 10.5, beta 5 is available to download at the Dev Center. If you are a registered developer having access to it, then you can grab iTunes 10.5 beta 5 from there.
Only iOS 5 beta 5 will be supported by iTunes 10.5 beta 5 and vice-versa. So if you have any one, you need the other one to sync your device.
Download iTunes 10.5 Beta 5 from here