The only way to buy a LEGAL copy of SHIII without the protection included AFAIK is by digital means.
Here are two options:
http://www.direct2drive.com/Search.a...=Silent+Hunter
http://store.steampowered.com/app/15210
If you don't already use Steam avoid it as you will find it a hassle for just one game. If you already use it, then you would already know if you want to purchase SH that way yourself.