Leonard A. Lauder, the billionaire son of Estée, has spent a lifetime scouring kiosks by the Seine and souvenir shops off Portobello Road for rare vintage postcards. He began his quest as a boy, in the 1930s. buying postcards of famous New York City buildings at Woolworth's with his allowance of 5 cents. Lauder is now 79, and his peerless collection is finally getting its day in the sun with "The Postcard Age" ($45), a book and corresponding show (through April 14) at the Museum of Fine Arts, Boston.