I may not be much help, because we tend to send postcards to the grandchildren from each city, with notes about what we've seen and think they would be interested in, and to avoid filling the suitcase with physical items.
But for those few things we do buy, we usually go to the market! One of our travel rituals, on the first day when we stock the apartment, is to look for a local preserve or jam that we don't see at home (skyberries in Stockholm, for instance). Once we've found one we really like, that's our gift to the select few, and a spare for us. A really good, really local olive oil is another possibility.
Sometimes it's just a good brand from a supermarket (in Portugal, tomato jam is popular and delicious), and sometimes--Barcelona is a good example--a big market, like La Bouqueria will have what you want, and be an interesting visit as well.
By the way...things like this are legal to bring in (sealed), but need to go in your checked bags, because otherwise you'll run into the 3-1-1 limits.