I recently purchased a package of 3 Bachmann On30 underframes on Amazon.com. I model in S scale and these should work fine to add underframes to older American Flyer cars I am converting to scale. Does anyone know where I can purchase more of these as I have need of more?  Also, does anyone know the size of the screws that are included with each package of 3 there appear to be 2 different sizes?
              Thanks much
                     Bob