You can buy old boxes from time-to-time on different sites, just as you can buy most anything. In addition, there are fake S&W boxes available, made in China. You can also buy blank labels and fill them in. Then you can try to sell the gun to some good-faith purchaser for lots more than it is really worth, like someone recently tried to do to me. It is called fraud, theft, cheating, counterfeiting, and a few other even less flattering terms. Fortunately, and with the help of some of the members on this forum, I exposed the fraud and was able to purchase the gun for its actual value rather than the inflated LNIB price they were trying to scam.
I think that purchasing a phony box for a gun, regardless of your motives, is a real poor idea. Even if you just want it for "sentimental" reasons, someone later on down the line is going to try to screw someone out of $ by claiming it is original.