The Sims 3 Hair Resource is a database of all custom hair for The Sims 3, both the original creation and the various retextures and alpha edits. You can use the search feature or the menu items to find the hair you're looking for.
Feel free to request hair (old or new) by providing a specific Sims hair creator name or an image. I do not create, mesh, retexture or in any way sculpt hair. Anonymous requests have been temporarily turned off to discourage requests like "Bieber hair 2013" and "side ponytail" (although that would be pretty radical).
To download, either click “original: donation” to go to the creator’s website and download it there, or if it’s a pay hair, click the “Free” link and you’ll end up on a separate download page. Just click the download button and voilà!