They are kinda pricey, but you can find them on eBay. You can get actual Xbox drives, but from what I've heard, a modified Samsung DVD-drive is the way to go. They are 16x, reliable, and work in an Xbox even without a modchip. Just attach the faceplate, and you are ready to go.
Here's one. I've been meaning to grab one for the inevitable day when my drive dies....
-Dave