Can anybody tell where it is available apart from the cisco site.I have checked at cisco but he kept all files locked. Help

Follow the link at the top right of the Cisco home page to login or register if you have not done so.

Your IOS download for the 851 Series Router is here: Cisco.com Login Page

Be aware that a maintenance/support contract (SMARTNet) is normally required to get access to ongoing IOS releases. If this rubs you the wrong way, have a look at Adtran router products. And no, I don't work for Adtran, I just find the company is a better value and much less hassle to interact with than Cisco for small to medium business applications.

even better value for the class:

Juniper Networks