If you wouldn't mind shooting film, Yashica used to make a camera called a Dental Eye. It was made for dental close ups, and they are getting cheaper. They used to cost hundreds but now you can find them on e-bay cheap if you just wait for one to come around. I have seen them go as cheap as 50.00 to 75.00 dollars and as high as several hundred. Another thing that I am looking at is tubes. You can attach them between your camera body and lens. You can use any lens in the bag for close ups. They are way cheaper than a lens and produce very good close ups....
cosmo