What happened to Robb Stark's crown after
He was assassinated at the Red Wedding?
We see the crown in AFFC when Jaime goes to break the siege on Riverrun, and the Frey heir has a whore with him who is wearing the crown, saying she is the "queen o' whores". (A Feast for Crows page 570)
Jaime then sends Ryman Frey away, and says that he may not take the crown. Later on, in Brienne's chapter page 637, we see Lady Stoneheart:
We also later learn that Ryman Frey was ambushed on his way home from Riverrun and killed.
Presumably, Ser Ryman took the crown with him when he left Riverrun (despite Jaime's command not to) and was ambushed and hanged by Stoneheart. Some small measure of justice for her, including recovering Robb's crown.
The summary of who is in possession of the crown is here.
This is not a full answer, since I don't know how canon it is, but Wikia has a picture of Lady Stoneheart
with Robb's crown:
We can assume that she got hold of the crown after the massacre of the Red Wedding.
I'm not aware of any specific mention in the books or show. But Robb was presumably wearing the crown at the Red Wedding, and given that
it would have been kept by one of his enemies. The most likely candidate is