it's a tube that fits on the camera body like a lens. you pop a mounted slide in at the other end, and basically take a photo of your slide. you can light it with flash or daylight, and use built-in metering or calculate based on guide no.
Originally posted by Juan Pena New ones start around $90.00, like the pro-optics (adorama) one. But there are quite a lot more. Try ebay, you'll see quite a few of them.
In ebay you can find many slide duplicators on sale, quita a few different brands and prices. That is where I got mine. On the net, do a serch with the words 'slide duplicator'.