The word garum doesn't speak for itself, so a short explanation is due. In orer to salt their food, Romans used (apart from sea-salt) a ready-made sauce, that was obtained by letting fish ferment in vessels exposed to solar, heat togheter with large amounts of salt and herbs.This proces, which was seen as a sort of corruption, resulted in a clear, amber-coloured liquid with a strong smell, but extraorinary culinary qualities. Nowadays a similar product is made in South East Asian countries. You should be able to obtain it in an oriental food store as 'nouc nam' or 'fish sauce'.