Are there any cyclocross/road bike rims that I could buy that will fit on my Cannondale F600 mountain bike?
Do you mean rims or wheels?

Many cyclists have a second set of wheels for their bike, because it's a lot cheaper than having a second bike. Buying a set of 26" disk brake wheels with narrow rims and putting slicks on them is a pretty common thing for MTB owners who commute on their bike. You might also choose a slightly higher-geared cassette for the road wheels. You will need a second cassette on the new wheel, and a second pair of disk rotors. Two complete wheels, in other words.

If you are talking about rebuilding your wheels with narrower rims that's expensive and I think pointless - it will not be much cheaper than getting a second set of wheels, and will limit your choice of offroad tyres. You can mount fairly narrow tyres on most MTB rims, especially on newer bikes as there's been a fair bit of work on making fat tyres work on narrow rims, so racing cyclists can have narrower rims (because they're lighter).

The cheapest option is to just buy skinny tyres, but that means swapping tyres every time you want to go off road. Which is why many riders buy extra wheels. Another cheap way is to get a second hand road bike (but be aware that that is a slippery slope and many of us have more than two bikes)