Penzey's has a great selection of difficult to find spices, and they have a knowledgeable staff who can help you find what you need. They are, however, fairly pricey - you're definitely paying extra for the selection and quality they've got. Unless you're loaded or a ridiculous food snob I would come here for special hard to find items (like berbere, mahlab, or Aleppo pepper), or super high quality regular spices (like cinnamon etc) for very special occasions.