Apple only keeps the last release of the current Pages available in the OS X App Store. This supercedes previous Purchases, and there is no Apple location to obtain older Pages versions.
In the past, on a Mountain Lion (10.8) system, the installation of Pages v5 expectedly failed, but unexpectedly, the App Store detected an out of date (App Store purchased) Pages '09 application, and offered to update it, which I accepted. This feature may no longer work on Lion and Mountain Lion.
If you have an App Store purchased Pages '09, and the second paragraph no longer works, you can not update that application further. You cannot reinstall iWork '09 DVD applications from a backup either, because they sprinkle dependent libraries into the System, and will not run standalone without those libraries.
Your last option is to obtain a retail iWork '09 DVD from Amazon or Ebay resellers, and follow that installation with the Apple iWork 9.3 updater, to arrive at the last application suite versions. You must never run this updater on iWork '09 applications installed from the OS X App Store, as it will destroy them.
If you right-click on your copy of Pages '09, choose show package contents, and double-click the Contents folder. If there is a Frameworks folder within, then you have an OS X App Store sourced Pages '09.