I know there are a ton of valid options out there, but I personally couldn't pay for shipping to get my old binders places, so I donated them to a youth shelter. There's only one even sort of close to me that's LGBT friendly so I donated them there along with some clothes that don't fit me anymore. I knew if they went there, some kids who really needed them would get them, kids who might not be able to get jobs to pay for them themselves.
I also considered giving them away online, probably through tumblr, to people who couldn't afford their own binders. But as I said before, I was iffy about whether I could afford the shipping.