There are plenty of lenses marketed as macro lenses out there like the Nikkor 60mm F2.8, the 105mm VR, the 200mm F4, and you also have 3rd party options like the Tamron 90mm or the Sigma 150mm.
The thing is, a macro lens is probably the LAST thing you will probably need to shoot macro.
Your primary concern will be light (more precisely, the lack of light), because shooting macro will yield a very shallow depth of field, you will be forced to shoot at very small apertures in order to get your subject in focus.
I would highly recommend getting a small flash like the SB600. Pair it with a small light stand and a diffuser box. It can also be triggered wireless by your D90, making it very suitable for macro work.
You can increase the macro capability of any lens with diopters or extension tubes:
-Diopters act like magnifying glasses that you attach as filters, you can also stack them to the detriment of optics. Nikon used to make some really good diopters but they really hard to fine now, and even more expensive. A good 3rd party solution is the Raynox DCR-250. It cost ~$55 and can also be used on many other device such as your p&s or digicam.
-Extension tubes are relatively inexpensive, can be stacked and have zero optics (meaning you don't degrade quality by using them). Look up for Kenko extension tubes as they provides AF and metering function on Nikon bodies. The Nikon extension tubes don't provide the proper electric contacts for modern lenses (AF-D or AF-S).
Buy the Raynox DCR-250, then maybe the Kenko extension tubes, and you'll realize you don't need a dedicated macro lens to reach 1:1 magnification. Don't forget your flash, then down the run, get a dedicated macro lens like the Sigma 150mm to couple with the diopters and tubes you already have for even greater magnification.