Wigs for kids is one and locks of love is another. I donated twice to wigs for kids. I also sent in my grandmother's hair, a beautiful, long silvery braid. They don't use those for the kids wigs, but they are able to sell them to make money to have the kids' wigs made.
wigsforkids.org
locksoflove.org
Looks like they're also on facebook and twitter.
M