I dont think you realize how mods work, mate. Gun mods like these needs to be intelgrated into the game. They need to be mentioned in files for them to appear on other NPCs and in the inventory of traders. Instead of worrying about where to get these particular gun mods (if I know Purke, he hasnt released them yet) just get the game, play it and perhaps get some mods that add new weapons, etc. There's alot, including some with the Mosin Nagant.
I Told you I haven't got the game yet those double barreled shotguns specially the Olympia from Black OPS are really awesome, But you know mods for example like LURK has to be downloaded and then to be played in game. I really need a good double barreled shotgun. Sorry if I act like a n00b.