Okay, your camera has a 1.5 FOV Crop. Which means a 50mm lens is going to be like using a 75mm lens. Works out really well when you want zoom capability. Not so good for prime lenses or wide angle lenses.
I found this one:
Sigma | 30mm f/1.4 EX DC HSM Autofocus Lens for Nik | 300306
Which would be equivelent to a 45mm lens on the Nikon.
I also found this one:
Sigma | 17-70mm f/2.8-4.5 DC Macro Lens for Nik | 689-306 | B&H
Which in my opinion would be a better choice. It can get as close as 8" to what you are wanting to photograph and that is dang close for a 70mm focal length (which would be equivalent to 105mm with the Nikon) plus it has an f/stop range of 2.8 - 4.5 which would be good in low light situations.
