If you visit France and take a look around any of the museum gift shops, you will see dozens of beautiful tapestries for sale. Most are made in France and all of them are frighteningly expensive.
Luckilly, before I went to Paris I discovered a couple of great tapestry retailers. If you are looking for quality tapestry designs from your home, The Tapestry House is probably the best resource available. They are recommended by the New York Times. If you are looking for reproductions of larger tapestries, such as the Unicorn tapestries, this is the place to go.
Although I love smaller design firms like Charles Rupert, this is definitely the most affordable place to find tapestries and they get their products from the same place that smaller design firms do (from the factories that weave them). Most of their products are made in France and they are identical to the tapestries sold in pricey giftshops. Those tapestries that are not woven in Europe are made in the United States.
If you are on the lookout for smaller tapestries, The Tapestry Standard is another great website. They carry a wide variety of designers--including (of course!) William Morris. They sell a lot of the smaller products that you will find in museum giftshops, such as pillows. Also, if you are interested in modern and casual designs, Tapestry Standard has a multitude of patterns to choose from.