First, you'll need a license to run Tiger on that iMac, i.e. you'll buy a retail version of Tiger. Then you can send that to Apple and have it replaced as the CD version. You might be able to directly order the CD version as well, I haven't looked.
However: It might be simpler to just run 10.3.9 on that computer. It's fine, isn't it? Tiger, on low RAM conditions (640 MB and less) is definitely slower than Panther.
Mac user since 1987. Running Mac OS X 10.8 Mountain Lion on a MacBook Air 11" & an iMac 27" and whatever's newest for my iPhone 4s, iPad 3 and AppleTV 2.
Apple Certified System Administrator 10.6, Apple Sales Professional 2008-2011, Apple Certified Mac Technician.