You can format the disks using Disk Utility on a retail install disk. The last system that will run on your machines is Leopard, 10.5.8 (that's the last update version, though usually the discs that you can get are a little lower than 10.5.8, but Software Update will get you fully current).
You can try Apple's store number 1-800-MY-APPLE or also check eBay or other sources for retail versions of the operating system. I emphasize retail, because there are also grey system specific discs which will not work on your machine, which you needn't waste your money on.
When you format the discs, you need to make sure that they end up with an Apple Partition Map, which you can tell by looking at the bottom of the Disk Utility main window when you have selected the drive of interest.
There are also documentation areas in the Leopard Area of Apple Support.