Where can I find replacement door knobs?
A quick trip to Home Depot or Loewes will find you a selection of similar door knobs with locks that should fit into exactly the same fitting.

You'll find either the ones that lock when you're inside, just pressing the button, or ones that take a key in the centre of the handle.

It should be a simple matter of unscrewing and removing both handles, and sliding the new handles in place through the existing bolt mechanism.

I've replaced a bunch of door handles that way, both locking and not.

Something like this: Locking door handle, and only $15.