Yes new parts can still be bought for the k10, i buy new part quite regularly from my local dealer. You're best going to scrap yards that let you walk around and remove the parts yourself, so that you can look at things yourself. I think the mot man either meant the constant velocity joint (cv joint) or he was just trying to get money out of you.
The only known issue with the k10 steering rack is that when the mounting rubbers become badly worn, they allow the rack to slide around in the brackets when you turn the wheels. I had that problem with my super s but that was solved quickly, i just bought a pair of new rubbers from the dealers, for i think about £10ish each.
The best advice i can give you at this point, is to ask somebody to sit inside the car and just turn the steering wheel slightly back and forth in each direction. While they're doing that get under the bonnet and watch the steering rack, see if it slides around in it's mounting brackets. If it doesn't then watch the knuckle-joint to see if it has excessive play in it.
If both these things are fine then there is nothing wrong with your steering and you should take your car to another garage for mot testing. (Y)