If your iMac is the iMac G4 "swivel lamp" or "snow ball" base style iMac, you need to tell us the year, screen size and iMac model indentifier number located somewhere on the outside of the iMac.
The reason we need this info in the first year of these models came with a CD disc reading only drive as a base model.
Later versions of this style came with disc drives that could read both CDs and DVDs.
At any rate, you need to look on eBay or Amazon for black OEM retail copies of OS X 10.4 Tiger.
No gray colored discs will work.
You're looking for black discs with a large, silver "X" on them.
But we need to determine if you need CD discs or DVD install discs.
Here is another more expensive source for the discs you need.
If you're wanting the original disk to use the Apple Hardware Test, the retail version doesn't have it. You need to find the original disk made specifically for your model.
Call Apple Technical Support. There is a 5% chance (if that) that they may have a disk to sell you. They'll need your serial number to determine the correct disk.