Eggs! They can bought from anyone, including idolators, without concern that they came via a nevilah or treyfa. The eggs themselves are identifiable as coming from a kosher species (or not) -- in terms of the shape of the egg itself, and more other ways of distinguishing one from another. But not if the egg itself is mixed. Also, if eggs are cooked together -- kosher and non-kosher -- the kosher eggs are still allowed. Unfertilized eggs are also acceptable, even if the hen sat on the egg for a long time. A droplet of blood on the egg won't invalidate it either - as long as you discard that droplet of blood. But if the blood was in the yolk area of the egg, then the entirety needs to be discarded.