You will need the retail iWork '09 DVD, which Apple has not retailed since Fall 2013. Apple has no downloadable full installer for iWork '09 either. The retail iWork '09 DVD is available from Amazon and Ebay resellers. If you go that route, and install the retail iWork '09 DVD, then for El Capitan compatibility, you should also apply the iWork Update 9.3 to it which provides the last versions of the individual applications.
Because the iWork '09 DVD installs component libraries into System locations, one cannot simply restore the iWork '09 applications from a prior Time Machine backup.