There are several resources you can use to find out many networks on IRC. Some networks do not wish to have public users, and therefore, you're unlikely to find their addresses anywhere.
You can search the drop down list in mIRC - this is a list of servers/networks that have been submitted by users to Khaled, and then have been included in the servers.ini file for mIRC. To view it, go to:File > Options > Connect
The drop down list is there.
You can also use the famous network statistics website, netsplit.de
or a dynamically created server list at IFIRC
Hope this helps.