I think the best way to adopt reptiles of any kind is to join herp clubs. Even if you don't live in the same state, join a turtle club and get to know the council members. Call them or email them and get on their adoption lists. Another way is to meet other people in your area who keep box turtles. They can perhaps help you get another turtle.
But of course you should be thinking long and hard about why you need another box turtle. They are long lived and require a lot of attention and care. I know people who have dozen and dozens of box turtles. And some tell me the fun is no longer there. It's a chore now to keep them. Tess