If you work for a company that has a SMARTnet contract on a 2800 series router (less likely since EOL was announced in 2017), you can:
Because of its enduring popularity in the used equipment market, one of the most frequent search terms among networking students and enthusiasts is Users acquire a used router only to find it missing the operating system, or they wish to upgrade the IOS version to support specific features like Voice over IP (VoIP) or Security (Firewall). Cisco 2811 Ios Download Free
Once you've obtained the IOS image: