I don't have any websites to offer, but the basic differences in hackles are the length and stiffness of the fibers along the stem. Like Danoinark said, the stiffer ones are used to float dry flies, and the softer, webbier ones are used on wet flies. You can get some of both on a good neck, with the highest grade/most expensive having a broad range of dry fly hackles from very tiny to mid-sized. Saddles come from farther back on the rooster and don't have the short stiff hackles for dry flies but have lots of the softer ones for wets such as wooly buggers, etc. If you're looking to tie something specific let us know and we can help.