Who gave the pen to Lecter?

We see a pen on a towel on his bed, next to the papers. It is definitely Chilton's pen because he can't find it in his pocket at the airport hangar. Whether Chilton forgot it or dropped it, I don't know. It looks the papers are Chilton's, because he is holding his pen in his hand. So I would expect him to take the papers with him while exiting the cell, which would make forgetting the pen, which is very close to the papers difficult. Or, with the joy of hearing Lecter will cooperate (so, he will maybe get a career boost for helping the Senator), he might have forgotten to take the papers and the pen and maybe Barney kind of didn't "see" the pen while preparing Lecter, per Lecter's whispering of his name and his reply as "hmm hmm".

Anyway, somehow he grabbed the pen and break it to pieces and took only its pin. To do that would require a period of personal privacy in his cell, during something like a shower or other biological need; he was going to be prepared for meeting the Senator. Taking the pin, he got rid of the rest of the pen, possibly threw it into the toilet.

He put the pin inside his mouth in a way it wouldn't drop or hinder his speech, before leaving his cell. Remember, he knows a lot about human anatomy, especially about the mouth I would say. Another reason for keeping the pin inside his mouth is that he probably knows they put a mask on his face with metal bars around his mouth while transferring him, so the pin wouldn't alert security check devices.

When the time comes, he takes the pin out off his mouth. I would also think that he is not only a psychology expert; he is a highly intelligent and knowledgeable person, who traveled the world (Florence drawing from his memory) and perhaps learned a lot. I would assume being such a serial killer would require some study and knowledge of "how to escape", in this case getting rid of cuffs. He is not an amateur who was busted on his first or second crime. He probably ate/killed a lot more who haven't been found yet.