There are no downloads, it is only available on disk, and no longer available from Apple. You would have to find it from a third party, or try to find a copy on a site like eBay.
A month ago, I took a 2001 iMac that had no OS on it to an Apple Store, and they loaded one on for me at no charge. No guarantee that your local one will do the same but it may be more convenient to try.
Try to locate the serial number on the surface of your mac:
Then you can look up this list:
Here is a list of iMac models and MacOS versions, listing the differences and MacOS versions: