Well, how big of an enclosure are you willing to provide? Will you be able to build an outdoor, screened-in enclosure for a box turtle? As stated, most adult box turtles for sale are wild-caught. Sliders, particularly the females, get large and do best in ponds (fenced-in, though, as sliders don't always stay put). I highly recommend adoption if you are considering a slider or box turtle, or any other common chelonian for that matter. A male slider would stay smaller, and there are lots for adoption.
In which state do you live? Turtles do best in climates that are similar to that from which they came, and it's illegal to own some species in certain states.
Katrina