So I've been trying everything I can to get the vendors in FO:NV to get the mods in stock for the Anti-Material Rifle for a few RL days now, and they never seem to come up, not even at the GRA shop in New Vegas. So I used the GECK to get the item IDs but when I enter them, I get a line one error saying the item ID is basically wrong, or does not match anything.

Is there any way around this?

Are you sure you're entering it right? When using console commands, the first two digits have to match the load order of GRA. So if it's 6th in your load order, your command should be something like 06xxxxxxx