Where do chefs get those baby greens and weird mushrooms? You always assume they have access to sources that the ordinary mortal cook can't get to, and, funny thing is, you're mostly right. Because you want half a pound of weird mushrooms, and the pros purchase a case at a time. But Cooseman's, specialty produce purveyor to the pros, has now opened its doors to the retail market, breaking down case packs so you can buy hard-to-find vegetables--cluster tomatoes from Holland, mesclun and baby lettuces, wild and exotic mushrooms-- even if you're not expecting 50 for dinner. It's open Monday through Friday from earlier than you'll be there in the morning until 3 p.m.; and Saturdays till 2 p.m. Advice is to avoid the chefs' rush; it's over around 8 a.m.