Ruined 2006 imac - Only allowed to reinstall Lion which is no longer availiable
If you don't have your original OS X 10.4 "Tiger" install media, you'll have to obtain a retail version of any system up to 10.7.5.; 10.8 "Mountain Lion"
10.9 "Mavericks" and 10.10 "Yosemite" require a 2007 or newer iMac to be installed.

I've found a couple of supposedly original retail installation media for 10.7 searching on eBay for $35-$40 (1), (2); just be sure to buy the retail version and not the recovery DVD unless it is exactly the same that came your iMac model.

You can try your luck downloading the OS from a non official source, but I wouldn't recommend that.