Most people will very happily give their turtles away for free. There are a lot of owners who have to move or otherwise decide not to keep a turtle. The bad owners either neglect them until they die or release them into the wild (which can ruin a local ecosystem). The good owners find people willing to take them, for how much I've seen, a lot of people will even give away their whole setup (like $400+ worth of equipment) just to be rid of them! Yikes! A place to start is here:
http://turtlehomes.org/, another forum that has an adoption link is:
http://www.turtletimes.com/forums/index.php?, and you can sadly even find turtles on Craigslist, in papers, and at the pound. Just make sure you check your state laws, some limit how many turtles you can legally own. I'm sure you could make some sort of reasonable statement that if they are living in your backyard, you don't own them, but just watch out, you don't want your turtles being confiscated!