Mail-order Nurseries
Mail order catalogues carry a large selection of plants, and the quality is usually good.
Be warned that some plants are shipped "bare-root" instead of in a pot--they arrive looking like a bundle of twigs and scraggly roots.  Don't panic; just follow the planting instructions.
White Flower Farm sells beautiful plants at premium prices.  They carry some unusual varieties and cultivars.
Send away for their catalogue--it is gorgeous.  Read it in January when the garden is frozen. 
Dutch Gardens is a source for good plants at very reasonable prices.  They carry some "heirloom" varieties (more every year).
Dutch Gardens is less expensive than a number of other mail-order companies which sell comparable plants. 
Van Engelen deals exclusively in bulbs, with many unusual varieties and cultivars.  This is a great source for old varieties of bulbs.
The minimum order quantities are relatively large, often 50 or 100 bulbs, so you might want to put together an order with some gardening friends.
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