What model G4 iMac do you have? You may be able to upgrade to Leop-ard 10.5.
Tiger install DVDs are in short supply. Tiger is no longer available at the Apple Store but may be available for $129 by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753). For other sources, do a Google search for MA453Z/A (10.4.6), MA190Z/A (10.4.3) and M9639Z/A (10.4). Also look on eBay. Be sure & buy a retail version (black with silver X) and not a model specific version (gray) (See http://discussions.apple.com/thread.jspa?threadID=1976068 Can I use another Mac's system restore disc on my Mac?.
Possible Alternative if you are currently using Panther 10.3 – Do a Google search for Tiger 10.4 Upgrade DVD . It will upgrade your 10.3 to 10.4, but it doesn’t contain the full retail Tiger installer. Should be cheaper than the retail Tiger install DVD.
After you install the base 10.4.x, update to the final Tiger version 10.4.11. Here's the link for the 10.4.11 combo update http://support.apple.com/kb/TA24901?viewlocale=en_US
Look at this link Mac OS X v10.4 Tiger Installing Software Support Page
You can also download the Tiger Installation and Setup Guide
When you upgrade the OS, do it via Archive & Install. See About the Archive and Install feature http://support.apple.com/kb/HT1710
Be sure to select Preserve Users & Settings. Also, repair permissions before and after the install.