They market appear to only sell the "Wireless Optical Desktop Pro" as a kit. You mean Microsoft doesn't know best for you?
They appear to only have an FCC license for the bluetooth kit (received in August 2002), not for the products individually. So I doubt you will find them sold individually, and if so I doubt its legal (but not many retail stores follow FCC licenses anyways). They have an older 27MHz wireless keyboard (Bluetooth is 2.4GHz) and mouse combo too, but I don't think that helps you.
Is it just me or is the 2.4GHz spectrum going to get pretty clogged soon?
I can't wait to see people sitting outside of companies and monitoring all their keyboard traffic (The Microsoft keyboards only have an official range of 30ft, but I'm sure you can pick up the signal from farther away with good equipment).