I see Super CD system cards for sale all the time, they're not rare yet.
Re the CD unit, I have the Japanese one and it took me a while to track down a working one that didn't cost a fortune. I'm not sure if the Japanese one is compatible with a TurboGrafx but I can't see why not, I'm not aware of any differences aside from colour. The black American CD units seem to be super rare, you might have better luck tracking down a Japanese one (assuming someone can confirm they are compatible with US consoles).
There's a few Japanese CD units on eBay but they're bundled with interface units/consoles and a bit pricey. I could have got you a working Japanese one for $70 if you asked about 1 week ago.