As you can to construct an entire gun, if you want a crude option. But this isn’t Japan. Obtaining a commercial silencer doesn’t require someone to smuggle them through international customs, just across state lines. They’re harder to get than a handgun, but not hard enough to get that a determined and capable assassin couldn’t get.
I think the point was, they’re hard enough to get that common street criminals don’t use them. They’re not something a criminal can just have their girlfriend straw-purchase from the local sporting goods store.