Although GNS3 is open source, Cisco IOS is not. You need to have a copy obtained by legitimate means to use. Please refer to this atricle on Cisco Learning Network which suggests your local Cisco SE (Systems Engineer).
Cisco themselves are soon releasing VIRL (Virtual Internet Router Lab) for free public use. See this atricle. It was demo'd at Cisco Live! in Orlando last month but looks like it's not quite out yet. (January 2014 according to this blog post.) It will include the necessary IOS built into the simulated routers.
I would highly recommend Packet Tracer. Its a great piece of software that has come along way in the past few years. You can download pre-built topologies all over the Internet and also custom build your own. GNS3 is also a great supplement, but the "switching" side can be a little tricky, so I would recommend using both applications in tandem.
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
Introduction Featured Speakers Luis Espejel is the Telecommunications
Manager of IENova, an Oil & Gas company. Currently he works with Cisco
IOS® and Cisco IOS XE platforms, and NX to some extent. He has also
worked as a Senior Engineer with the Routing P...
In this session you can learn more about Layer 3 multicast and the best
practices to identify possible threats and take security measures. It
provides an overview of basic multicast, the best security practices for
use of this technology, and recommendati...