I just checked a few of the Lionel screw sets offered on the bay. Most are common 6-32 and 4-40 which can be had at any hardware store. Get bright plated (not stainless) and they can be blackened with gun blue. Save money, buy boxes of 1" long screws plus a screw cutter and cut to size needed. 4-36 and 3-48 will likely have to be ordered from a specialty fastener outfit unless you have access to a local hardware store thats been in town for say 100 years. They might have some buried in the back storeroom somewhere.
Pete