If you own a physical Cisco 2811, you can extract the IOS image via:
This report provides a basic guide on downloading a Cisco 2811 IOS image and setting it up on GNS3. Successful completion of these steps will enable you to simulate a Cisco 2811 environment for educational or testing purposes. Remember to comply with all licensing agreements and terms of service provided by Cisco. cisco 2811 ios image gns3 download for 19