The most common Coach gun being used in CAS is the Stoeger. It is an internal hammered shotgun with 20" barrels. You can pick one of these up for about $300. There are some new hammered guns about, but they are either quite expensive or are Russian or Chinese made, and the quality of these is spotty. The EAA gun is Russian made, but the external hammers (which are quite skinny) only cock the internal hammers-they don't strike the firing pins. Rossi made a nice mule-eared Coach gun, but no longer. You can still find them used for about $400.
Hammerless guns were available in the "Old West".