Depends - best bet is to start with the maufacturer of said shells. Can you identify who made them?
Next best bet is to get parts from a similar model - e.g. Accurail sells separate parts for most of their kits. You'd likely have to buy one to determine compatibility, and then could order the required parts.
Last option is to scratchbuild the underframes from wood, brass, or styrene.
Weights can be cut from sheet lead or similar dense metal. Alternatively, if you can live with permanently loaded hoppers, the weight can be concealed in a load.
Good luck, and welcome to The Gauge!
Andrew