I'm fairly certain those nuts are actually welded to the chassis. As such, they are not available separately (ie: you aren't supposed to strip those out).
Due to the special tapered shape, which centers the upper mount, replacement with a generic item is probably not an option. Your best bet would either be to heli-coil the existing nuts, or to drill them out and use a larger diameter and longer bolt, and add a nut on the top side. I'm not sure how accessible the top side is, though. I suppose another option would be to harvest them from a junkyard B6. I wonder how the dealership would handle this. I suppose the full, official fix would be to replace the complete inner wheel housing: the part of the chassis sheetmetal which includes those nuts.