are used radios available for a 2011 sonata
Yes , but why do that ? A sound shop or even Best Buy can put in a new one with the latest features and a warranty for just a few more dollars.
Yes, any junkyard has one or 2. The problem is they are the same 8 year old radio you already have.
Replacement radios are not expensive and they are brand new AND, as @VOLVO_V70 said, you can get them with a bunch of cool features not on your original radio.