On Saturdays, you will find vendors not only in the galleries at Plaza del Ángel, but also in the central courtyard, the hallways and on nearby streets. It is a great spot to find interesting pieces and colorful pieces. Look for china, old wine bottles, disc telephones, retro posters, phonographs and much more.
It may be difficult to bargain, though. The galleries at the market are very well established. If you are looking for a great deal, you may want to try other markets in the city.