OS 7.5.3 is the latest full-install OS download available from Apple. All later versions are updaters only, and require a specified pre-exisiting OS version to already be present. RE OS 9, this mans that all OS 9 downloads from Apple require some version of OS 9 to already be present.
You will need to acquire an OS 9 Install CD in an adequate version for your model, install that, then apply updaters as needed.
What specific model iMac G3 do you have? What is its processor speed? This article may be able to help -
Also, if the serial number lable is still on the machine, the model info, including processor speed, is shown on that.
The reason to determine the model - all Macs have a minimum OS which they can use. There's no sense in acquiring an install CD for a version too early for that model.
That model is also referred to as the Summer (2000) model.
It originally came with a model-specific version of OS 9.0.4. It can use that original OS 9.0.4 Software Install CD, but finding one of those will be difficult. It can also use a retail OS 9.1 Install CD or a retail OS 9.2.1 Install CD (all retail OS 9 Install CDs have a white label with a large gold 9 on them, and do not refer to any specific model on the label).
Some users who installed the latest firmware update for models like that later reported that their original OS 9.0.x Software Install CD would no longer work. Since there is no easy way to tell whether the machine has had the firmware update, you shgould probably look for either a retail OS 9.1 or 9.2.1 Install CD.
Places like Hardcore Mac sell such retail disks, but at a very steep price. Places like EBay are more affordable alternatives, but be sure you get exactly what you need.
You can also try contacting an Apple User Group - most large cities and universities have one.