Just one block off Campo de' Fiori, and therefore not much of a detour on your way to Roscioli, is a very fine little wine bar called L'Angolo Divino. The proprietor will give you good recommendations for a glass of wine no matter what you might be interested in, or at what your price range is, and also suggest good pairing of particular snack foods to accompany the wines. Or have a glass of wine with the complementary taralli snack crackers. The L'Angolo Divino street angles off Campo de' Fiori just where the piazza meets Via dei Guibbonari, the street where Roscioli restaurant is.

http://www.angolodivino.it/it/homepage