Hi Shabaz,
You can't just download a legitimate IOS from the web. You need to have a router that has an active Smartet contract on it in order to be able to download a legitimate IOS.
As with most things on the internet, any of the popular search engines will help you...
If you already have any of the software based routers such as 2600/3600/7200 then you can tftp the IOS from them and use that in your GNS3/Dynamips lab.
HTH
Kevin