iTunes Download Page
http://www.apple.com/itunes/download/
This should list the version for whatever OS you are using to access the page.
Your music should all be in the /Volumes/
Boot Disk/Users/
Username/Music/iTunes folder along with your library files. Or your music resides in whatever directory you put it in or told itunes to use as your default directory if you changed it.
When you install the latest version you may have to wait a while as it rebuilds the library database, organizes the music folder, updates the album artwork etc...
Alternatively you can install the version from your original Mac OS X install disc. But it will be an older version a may not be compatible with your library file if you have been regularly updating iTunes. Which means adding all of your music and making new playlists.