Here's another one I found quite good:

A woman goes into her local music store looking for an old record; behind the counter is small young boy.

She says: "Excuse me sonny, but do you have Jingle Bells on a 7 inch?"

He says: "No, but I've got dangling balls on a 9 inch."

"That's not a record is it?"

"It is for a 10 year old."