You can download Cisco IOS for GNS3 from here.
Cisco Router:
Download: Cisco IOSv 15.6
Download: Cisco IOS XE CSR1000v 16.8.1a
Download: Cisco IOS XRv 6.0.1
Download : Cisco 7200 124-24.T5
Cisco L2 Switch:
Download Cisco IOSv L2 15.2.1
Download : Cisco IOU L2
Download : Cisco IOU L3
Download : Micro Core Linux Version 6.4
