IOS is a licensed software product. You have to buy it.
I believe you can purchase it directly through Cisco, either as an individual licence (per router/switch/device), or you can purchase a "SmartNet" contract that entitles you to updates for a year.
You can also buy either from someplace like CDW (www.cdw.com - search for IOS or COSCO IOS ..).
Good Luck
Scott