Leopard disks are a seller's market, since Apple does not sell it anymore.
If you have an Intel processor and at least 1 GB RAM, you can step up to Snow Leoaprd.
Regardless ... PLEASE do not find a torrent download of Leopard. Not only are they selling illegal product (many installs of one license) but they also add hidden code to report your activities to their parent server.
I'd found a website article stating that Apple could still sell you a copy of 10.5 Leopard; however, when I called their number I did reach Apple, but all they had available was 10.6 Snow Leopard. If you keep an eye at local used computer shops you'll sometimes get lucky and find a copy of it for $150-190. I ended up getting a copy of 10.5.6 off eBay for ~$130 USD. Averages $150 as of 01/2013.
So we know more about it...
At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...
Model Name: iMac
Model Identifier: iMac7,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.4 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 6 GB
Bus Speed: 800 MHz
Boot ROM Version: IM71.007A.B03
SMC Version (system): 1.21f4