Sure, you can pay premium prices at snooty kitchen shops and department stores for those little accessories that every kitchen simply must have -- cheese graters, Bundt pans, grapefruit knives, cookie sheets. But why? For more than 23 years, the elegant Mrs. Blackwell has been peddling gently used housewares at bargain prices in the city's most artfully arranged thrift store. Drop the glass coffeepot this morning? Don't buy a new coffee maker; just hunt through the array at Mrs. B's, where there aren't just replacement coffee pots but also coffee tureens, coffee presses, tea kettles in every style and color, china teapots, coffee Thermoses and coffee canisters. Cocktail shakers share shelf space with cookie jars and rolling pins; elsewhere, there are Barney cake pans, Fire King custard cups, copper double boilers (the kind that cost a fortune at Williams-Sonoma), TV tray tables, deviled egg dishes, ice cream molds, Coleman camping lamps, electric irons and an occasional trombone. If you can't find a kitchen treasure in here, it doesn't exist.