Hi,
IOS's are available via the Cisco site, but are not necessarily free.
If you don't have an existing support contract or access to an existing account with IOS access privilages then you may need to pay a fee to upgrade.
It's also a good idea to register at the Cisco site to gain access to additional areas. Registeration is free.
It would also be a good idea to check the memory on the 2511's to ensure it meets the required spec for the IOS you want.
If the IOS is required for a lab (rather than a production) environment then you may wish to check e-bay for Cisco IOS's and/or memory upgrades.
I believe that other sources such as 'IOS Hunter' are available as well, but I'm unclear on the legality of these. Perhaps someone can clarify the situation?
This may seem like a pain to 'simply' set up the lab, but IMHO this is one of the benefits of using physical kit - you have to learn how to perform tasks required in real life . Annoying at times, but also educational.