NT OPerating System software

NT is no longer "for sale" per Microsoft. You can purchase it on Ebay, etc, but beware of licensing issues etc. As I understand it (and I'm no lawyer or MS licensing expert), if you have a Windows 2000 license, you can apply that to NT and use it.
That said, 2000 is a significant improvement on NT and I would recommend you use that if possible.
For list prices, check out MS's official 2k site:

http://www.microsoft.com/windows2000/

Your local CompUSA, OfficeMAX, or most local computer stores should have it for sale. You may even be able to get it slightly discounted (especially if you buy it with a system).

Good luck...