One long-shot thing:
If you are in North America, you can phone Apple Sales at 1-800-MY-APPLE with your computer's serial number in hand and ask for an operating system specialist. Ask if they have replacement system install/restore disks for your model--the serial number will let them make a specific search. These are subject to stock-on-hand so don't be surprised to hear a "no." The older the PowerMac, the less likely they are to have replacement gray disks that work with your specific variant.
If they say know, ask if they have full retail Leopard disks. If so, your search is over.
I'm hearing mixed messages about Apple's remaining stock of Leopard. Some say they are out but other still occasionally report that they were able to get a retail disk that way. The call is free and takes minutes to find out.
If you are not in N. America, follow japamac's recommendations. However, in the open market, be sure to specify you need a full retail install disk, not the gray ones that came with any old Mac model. Because of high demand for 10.5, the scam artists are hard at work, so caveat emptor!
A proper Leopard retail disk looks like this:
If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:
MB427Z/A Leopard 10.5.1 install DVD
MB576Z/A Leopard 10.5.4 install DVD
MB021Z/A Leopard 10.5.6 install DVD (single user)
MB022Z/A Leopard 10.5.6 install DVD (5-user family pack)
Installing Mac OS X 10.5 Leopard
Mac OS X 10.5 Leopard Installation and Setup Guide
After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update