While I have (sadly) never been to La Plaza for lunch, I do shop here quite often! It's the best place to go for unique ingredients for a decent price. I visit La Plaza when I want to make verde salsa (tomatillos are abundant here) or I need already ripe avocados. They also sell pre-made salsa's and guacamole, which are really delicious.
I don't speak Spanish, so I don't always know what products are--but I love browsing the aisles and checking out the international products. La Plaza also sells a big selection of Mexican snacks and beverages, including glorious MEXICAN COCA-COLA and every Goya product you could want. The staff is always friendly and (everyone I have encountered) are bilingual.