The only way you MIGHT be able to find one is to see if ASUS has any stock on a replacement I/O shield, or find one by itself or with maybe a bad board combo, on ebay. I don't think you'll find one any other way and honestly if you could make one yourself if you are handy at all and have access to any tools for fine work, or not worry about it at all. If this is a mini PC it might not be the worst thing in the world to have another rather limited area to expel heat since likely it has no top vent and probably only a rather small intake with little static pressure. Unless there is a specific reason to worry I'd either not worry, or make something custom that you could glue or epoxy to the case I/O area.