Finding the drive is the easy part. Any of the hard drives on this page:
http://eshop.macsales.com/search/3.5InternalIDE/ATA
will work. Your iMac uses IDE/ATA or PATA drives, not the later SATA drives. PATA drives are getting harder to find and the vendor I linked is one of the few who still carries them. Older iMac G4s may or may not recognize disks larger than 128GB. If yours doesn't, you'll have to partition any new drive so there is a Mac Extended HFS+ partition that's about 125GB and leave the rest unformatted. It's a hardware limitation common 8 years ago.
The next problem is that the iMAc is one of the most challenging Macs to take apart and service, and you have to go deep inside to get to the hard drive. You literally enter at your own risk. There is a significant chance of the computer not working afterward unless you can find someone who knows how. Apple does not work on these any more so you'll have to find a person or firm with Apple experience.
You are about to find out how expensive misplacing the system disks (CD or DVD) can be. To reinstall the OS, you need either the original gray disks that came with your computer or a full retail install disk set. The later for Tiger is no longer available from Apple and those copies on the market are going for up to US$250. That greatly exceeds the value of a G4 iMac on the used market. There is such demand for Tiger that the criminal element has sneaked in; I've seen blatantly illegal version of Tiger offered on auction sites. They are as expensive as the real deal and may not work.
It's remotely possible that you can call Apple and give your serial number and they can find a gray disk set that matches. However, the are out of those sets for computers made in 2004 so it's a long shot.
Your best bet may be to connect with a Mac User Group. They often have people who do
pro bono service and may have system software as a reduced price compared to the price-gouging going on in the open market. This page can help you find a group near you:
http://www.apple.com/usergroups/find/