Leader was a trade name used by C.S. Shattuck, Hatfield, Mass (1880-1890), Hopkins & Allen, Norwich, CT (1868-1915), and Harrington & Richardson, Worcester, Mass (1874-c.1980).
Numrich doesn't show much for Shattuck or Hopkins & Allen. Do you know who made the revolver. You might be out of luck for parts. Unless you find one on gunbroker.com, gunamerica.com, or in shotgun news. Do you have any of the 30cal rimfire ammo? If you do I would look at keeping it intact as it may well be worth more than the gun itself.
Seeing as this is a trade name the revolver may not even be worth the price of a new cylinder if you found one anyways.
Are you sure it is a 30 cal. Under Hopkins & Allen could find nothing about a 30 cal rimfire just a 32 cal and a 38 cal. Might you post some pictures of this unique revolver?
Sorry I could not be of more help. Let me know if you find the part.