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.
well since im new to mac and since im using this imac just for fun my main pc is windows 7 desktop would you recommend i install tiger or leopard also my cpu is 1ghz andf my ram is 1GB