All of these are great ideas: the fiber festivals, posting on localharvest.org website, and selling it through
eBay. And, I have a few more ideas, too. It just depends on how aggressive you want to be about marketing your fiber.
I don't spin but have plenty of friends who do. (And they're always trying to get me to start!) I've been knitting for over 10 years and have just started learning crochet.
My friends purchase their fiber from all of these sources. We also have purchased fiber and yarn at local farmers markets and craft fairs.
(YarnCon is being held here in Chicago this weekend and we're all sure to by something!
http://www.yarncon.com/)
If you're anywhere near a university or college town, you may also want to post some flyers or something there.
Knitting and spinning are very popular these days with people of all ages (including guys!) and a local source of good fiber is always much appreciated.
Also ask at any local yarn shops. You may be surprised at how supportive they can be about selling your fiber and yarn for you. When I say "local yarn shop" I DO NOT mean the big craft stores like Michael's, JoAnn, etc.