Generally known for consumer products, Casio decided to make a stab at producing some professional level instruments. Well in '81 I guess they actually produced the CT-101 which was like a super simple 8 voice analog keyboard with woodwork and all. And then the 1000P a little later I think. But the CZ series is the most well known, using what they called 'phase distortion' synthesis. Digital synths with fairly decent features and interesting sounds they are always a bargain for those wanting to experiment with yet another variety of sound synthesis.
After making the FZ-1 sampler which made a decent impression on the industry, and then the VZ-1 in '88 they returned to primarily consumer oriented products again. In '86 they had produced the SK-1, touted as the world's cheapest sampler. They're cool..especially when you get them in the thrift store for a buck 50 like I did :-). It wasn't until '87 I think that I actually played with one of these when a kid in a shelter home I worked at got one for Christmas. People were blown away by how nice the piano sound was in that tiny thing. Hehe. They were all like looking at me like I was nuts for paying a grand for the Kawai K3 I had at the time, whose piano wasn't too hot of course since it was an analog hybrid synth.

Here's a Landmark History Page from Casio to 2009 last I checked. In more recent years the XW-G1 and XW-P1 digital synthesizers have been an interesting product for music production. I think they first appeared in 2012 or 2013.

FOR CASIO PARTS (including the newer stuff) you might start with Casio Junkyard or PAC Parts or Full Compass ( if you don't have success with casio themselves at 800-706-2534)

Products:

1000P CZ-101 CZ-1000 CZ-5000 CZ-1 CZ-3000 SK-1 Sk series HZ/HT series FZ-1 VZ-1

Also here's an interesting page on modifications of earlier MT series casios etc. and another at Warranty Void!