Try Amazon. Be aware of the following:
1. Unless you have at least 1GB of RAM you will be disappointed with the performance of Leopard - go for Tiger instead. Although Leopard supports 512MB it does not perform well.
2. You need a 1.0GHz or greater processor for Leopard.
3. Make sure that you buy a retail package, not a disk intended for a particular machine. These abound on eBay, but will only work on the model they were first supplied with. As that model of iMac was never supplied with Leopard, a machine-specific disk will not work. The same goes for Tiger.
These OSs are very old, and unsupported. You might get better results by running Linux.
Oops; just noticed that you said not Amazon or eBay. I don't think there are any other reputable sources, so that increases the Linux recommendation.