Is my black-bordered foil from 8th edition a counterfeit? Where can I find more copies?
According to MTG Salvation:


  All foil cards are black-bordered, even those from the last white-bordered core sets, except those from Unhinged which are silver-bordered.


A black-bordered foil Thorn Elemental is valid anywhere any other Thorn Elemental is. You can find it, for example, here. Some sites just don't show separate images for foil versions of cards.