One of the more reliable wine shops and tasting rooms in San Gimignano, Enoteca Gustavo carries a well-chosen selection of Vernaccia di San Gimignano from the main local producers alongside Chianti Classico and other Tuscan reds. The staff know the wines and are happy to talk through the differences between producers — the range from lean, unoaked Vernaccia to the richer riserva-style versions is wider than most visitors expect.
You can buy by the glass and sit for a while, or pick up bottles to take home. It's a better introduction to Vernaccia than most of the restaurants, where the wine list is sometimes an afterthought. The shop is a short walk from the main piazza, in the quieter streets toward the western walls.