No one here will actively encourage you to pirate the game - however, assuming you have the instruction manual and a valid cd key. There is very little chance anyone would accuse you of committing any crime, since the game manual and the cdkey are your property and the cdkey gives you the entitlement of playing the game.
Not sure if posting links to torrent sites would be allowed, so instead I will post some search queries that you could try at your own risk.
Google: Proxy Bay or Pirate Bay Proxies
(I am in the UK, and my ISP blocks pirate bay)
While Pirate Bay does facilitate the supply of pirated material, the context in which you wish to use it is simply to get a replacement disc,which you can prove you are entitled to. Which means this is probably one of the more legal scenarios for downloading ISO's.
Note - I would be very surprised if Neverwinter Nights Deluxe edition is not on pirate bay.
Of course the 100% legal option is to buy the game again from GOG.com
I have my own reservation about that however, because if you bought the game once before, can prove that you are legally entitled to the game (eg: have the box and cdkey etc), then buying the game on GOG.com seems like a waste of money. Even if it is 1-10 dollars, out of principle, you shouldn't have to pay again for something you own already.
Of course, none of this applies to me, cause I just run nwn from the same directory on my computer, and its always been transferred to new computers etc
Not even sure when the last time I saw my install disc was, simply because I don't need it.