When you ask if you can purchase all the left over rocket souvenirs, you are told they are in the closet behind the counter, and that you can buy the key from his shop. You can find the radioactive isotope at the DinoBites gift shop in Novac.
Its properties compare to the rocket souvenirs sold at the Dino Bite Gift Shop for the Come Fly With Me quest when buying 5 because the same agent is inside the souvenirs. You will not be able to drop this item if you pick it up after the REPCONN quest is complete.
Isotope-239 igniting agent is a fuel used for the rockets at the REPCONN Test Site in the quest Come Fly With Me.
