Bolt buffs are legion around here, and those lucky enough to hear of these two rare open-to-the-public design houses come to adore them. At Sal Beressi Fabrics, vintage-cloth lovers can spend a whole day checking out the cotton and rustic wools in the Midcentury Modern aisle, plus the 200-plus bolts of fabric ranging from the late 1940s to the early 1970s. Best of all are the 40-percent-below-list prices. If silk is your thing, head to the Silk Trading Co., which draws residential, restaurant, and clothing designers from all over the world. With fabrics for as little as $10 per yard—and gifted staff who are happy to help you make selections—there’s room for amateur decorators, too. News flash: The store just acquired a collection of luxurious silk taffeta, hand-embroidered with delicate butterflies and traceries of flowers. Sal Beressi: 1504 Bryant St., 2nd Fl., 415-861-5004; Silk Trading Co.: 1616A 16th St., 415-282-5574, silktrading.com