Where can I find the Itunes version 10.5

Ok so I have a Mac OS X version 10.5.8 laptop and I just got an Iphone 4 and I tried hooking it up the the laptop and it says I need The itunes version 10.5? Where can I download it?

Added (1). And it's not on the itunes website.

Added (2). And I tried the software update. Nothing.

Just go to www.itunes.com

Go to system preferences and click software update. It would search for updates for your iTunes and thus you can update it.