Umpqua has the mose extensive library of flies that I have found:

http://www.umpqua.com/flygallery.htm

Use the search tools at the bottom of the page to navigate.

Orvis also has a lot of flies on its site as well as the interesting
feature of including most of the Fly Pattern Recipes:

EXAMPLE: (March Brown) --
http://www.orvis.com/store/product_c...147&shop_ id=