Sherry vinegar
Vinegar made from sherry and aged in casks around Jerez in Andalusia, rounder, nuttier and deeper than ordinary wine vinegar.
Sherry vinegar is sold in well-stocked supermarkets, delicatessens, Spanish grocers and online, but often not in a basic supermarket. If you cannot find it, a good red wine vinegar comes closest, though it lacks the nutty depth and mellow roundness.
Substitute:
Good red wine vinegar, ideally with a few drops of dry sherry, comes closest but stays sharper and flatter. Balsamic is too sweet and darkens the dish, so it noticeably changes gazpacho and vinaigrettes.