There really is no web site, per se. It's mainly just common knowledge: the only iOS that is signed by Apple is the most current for that device. Once Apple releases the new iOS version, it generally is 24 to 48 hours and then all the older versions are no good.
Most tech sites keep their visitors up-to-date on which iOS is the most current. Or, you could visit our Hacking section and check out the threads there. In addition, we have a News section that'll tell of upcoming iOS releases.
In a nutshell, the current iOS is the only one signed and authorized by Apple. So, for the iPad1, it's iOS 5.1.1. For all other iPads (to include the Mini), it's iOS 6.0.2.
Hope this helps.
Marilyn