I had a bit of a Google (I'm having a slow arvo) and found a company called Optolong which seems to produce clip in filters for some Nikon cameras. The reviews of quality are mixed (though not sure how many are first hand) but it seems some are saying they're infringing some of Astronomik's patents. Though if Astronomik don't make Nikon clip-in filters then you don't have much choice I guess...
Look on the left, about half way down, on this page and you'll see the clip-in filters:
http://www.highpointscientific.com/b...ronomy-filters
http://www.cloudynights.com/page/art...ison-wit-r3008
http://www.cloudynights.com/topic/52...-poor-quality/
Interestingly they make clip-in filters for full frame Canon too, which Astronomik didn't do last time I checked.