Dean, as well as Nikon fit, you could also look at M42 fit, which, although they require an adaptor, you do not lose any image quality, because they do not require an extra element to preserve infinity focus. If you went down this route, then you could also make use of some very cheap extension tubes and bellows units, also in M42 fit.
With regard to lenses, I believe that Tamron, Vivitar and Takumar (Pentax M42 fit), all made decent macro lenses.
Andy