Do you have any other Lee pistol die sets? If so, you can pull the decapping rod out of one of them and put it in your .40S&W sizing die and get back to reloading.
Lee will probably send you a new one and may or may not ask you to send the old one back. Sometimes they only ask for a picture of the broken part. How did you break it anyway? I've only lost the pin out of one Lee die and that one was pulled out when it wedged into a very tight flash hole. I've broken one RCBS pin when I turned the turret with the ram up.
Other than that all my dies have the same decapping pin that came in them from the factory, some more than 20 years ago.
Lee decapping pins are not replaceable by the average user. They are pressed into the decapping rod.