Do you want someone to either make Flies for you so you can either use them or to sell them or do you want to sell Feathers to Fly Tyers as there's probably a Million or so Fly Tyers on many sites all around The World who are always after feathers.
The only problem you probably will have is if you join a site you will have to become a Corporate member to either Sell Feathers or Flies.
Also if you advertise to sell them on Sites like Ebay The Fees are quite High.
Are you looking for Fly Tyers? Have you tried Facebook and similar social networking site? I myself just searched the term "Fly Tyers" in to the Facebook search bar and it produced hundreds of results. Facebook business pages like Fly Tyers are always welcoming your business and are often sole traders who will produce great quality work for you, you can also connect with them on a more personal level, assuring mutual benefit.
If you're after something to actually make you're flies with, eg. Fly Clamp, then just heck out eBay or local classifieds!