Hi Marty, the best place to look is Ebay the sound cards come up quite often, you have just missed one of the rare ones No4, as the KN 6000/6500 also used the boards numbers 1 and 2 were easy to get as their sale time was longer than for boards 3 and 4 so the last 2 are much more difficult to find. remember to set your Ebay search to world wide as this will offer a better chance of finding what you are looking for.
Some people have said they are not worth having,I must say I disagree and have all four and they still offer the very high sound quality that NX sound gives.
Best wishes and good hunting regards John.