Puppetswar has a wide range of heads (that's also mostly
40k focused). I know they have plain skulls and skulls with Roman legionnaire helmets. I can't say about their exact size. They're made with Space Marines in mind, so their size should fit Sigmarines as well, but I can't tell from the pictures if they are larger than a modern
GW head and would work as skull masks.
Tomb Kings have oversized heads which fit Sigmarines as skull masks quite well. I used one from Tomb Guard to convert my Knight Questor. The only downside is that they don't come with a neck and you have to make one yourself.