If there's one thing for which Games Workshop can be counted on, it is skulls, which you can scarcely miss, even on their terrain boards. Forge World used to make a set of slightly larger than 28mm resin skulls, though I don't know if it's still available.
Irregular Miniatures sell packs in their Pirate range, which are actually made by Eureka. There's also a pile of skulls, and a circular skull and crossbones motif, though these may be a little unrealistic for your purposes.