At every workshop I lead, someone wants to know where I got that great eyecup on my Nikon D300 and D200 cameras. Well, it's not a one piece eyecup. It's an assembly of parts originally meant for different cameras.
The cool thing about this assembly is that it fits more than the Nikon D100, D200, D300, D70, etc. It also fits the Canon 30D and perhaps even more. (it's been reported by reliable sources that the eyecup assembly will also fit the Canon 20D!)
I can't guarantee fit on any particular camera so purchase the parts at your own risk.
(If the Eyepiece Adapter fits your camera, then you're all set; it's only about $4 so it's pretty cheap to test it out.)

Here are the links to the parts at the B&H website:

DK-22 Eyepiece Adapter F3HP $3.95

DK-3 Rubber Eyecup  $11.95

Finder Eyepiece  $16.95