The Pages version available at the Mac App Store requires OS X 10.7.4 or newer, so you can't purchase it.
If you don't want to upgrade Mac OS X now, you can buy iWork '09 on a DVD at Amazon or eBay, and install it > http://www.amazon.com/Apple-MB942Z-A-iWork-VERSION/dp/B0014X2UAK Then, open > Software Update, and install the most recent version for Mac OS X Snow Leopard.
If you want the latest Pages version and all its features, you have to upgrade to OS X Lion or Mountain Lion. Open the Mac App Store and purchase OS X Mountain Lion (if your Mac is not compatible, it will give you an error message).
While it's being downloaded, make a backup of your data with Time Machine and check that your applications are compatible > http://www.roaringapps.com
When the download finishes, the OS X Mountain Lion installer will appear, so follow the steps. After upgrading, purchase Pages