I hunted around for Ribena a few times and was surprised they didn't have it Marché Hawai. Sometimes I get a French cassis syrup which is pretty similar and easier to find. I can get it at Co'pain d'Abord, for example. The brand I have at the moment is William.
Just a tip - there are quite a few British products available in some of the deps in the Parc Ex area. A Sri Lankan dep there is my source for Marmite and Hobnobs and I've seen Ribena there (and the classic lime cordial, I forget the brand name but it's the old school one)