How to downgrade MacOS in iMac?

A Late 2009 iMac can run up to macOS 10.13 High Sierra.

Apple has downloadable installers for macOS from 10.7 Lion through Big Sur OS 11 here:

How to get old versions of macOS - Apple Support

First, see if the recovery partition survived the wipe, noting the boot options listed here:

About macOS Recovery on Intel-based Mac computers - Apple Support

If there is no recovery option due to the wipe (seldom a good idea with Macs) then you still hav ethe option of creating a bootable installer from a flash drive:

How to create a bootable installer for macOS - Apple Support

See if this helps.

https://www.macworld.com/article/673171/how-to-install-older-versions-of-macos-or-os-x.html#:~:text=%20How%20to%20install%20older%20versions%20of%20macOS,downloaded%20you%20might%20think%20you%20can...%20More%20

Although probably not an option, I'll ask anyway: you don't by chance have the original install media?

And yes, you need a startup disk. That is the basic OS. Yours probably came with 10.6.3.

You can try ebay although my experiences there have not been good - I heard that a major portion of software sold there is pirated or copied.