6S was discontinued by Apple about 5 years ago, so if you can find a "new" 6S, it has been sitting in a warehouse for a long long time. Whether or not the battery would even function at this point is an open question.
You can purchase used phones from Ebay.com, but that would be a "buyer beware" situation at best. Few users sell phones that are working just fine. Don't buy anything unless you have a firm money back guarantee in hand.
Your best bet would be a reputable local shop that deals in used phones. That way, you can actually see the phone before you buy. Again, a money back guarantee is a must.