Need more explanation of your mechanics term "knuckle" is. My definetion would be the part that holds the wheel bearing and attaches to the upper control arm. There's nothing that goes bad on that that would need replacing. Other than bending it in a collision or something. The upper ball joint attaches to it, which is available as a whole upper control arm. The lower ball joint attaches to it, which is available seperately. The out tie rod attaches to it, which is available seperately. All of these things can cause a clunk.

If he's talking about the knuckle (u joint) on the steering shaft, I'm sure you can get one new somewhere, but if by chance you can't, hit up a junk yard or a board member.