As above, you have to be sponsored by a government dept to get it (for instance in my case it was BIS), costs approx £50 for SC level.
Contractors do it by being getting a job within a year of leaving the last thus keeping the clearance, which can then just be renewed (I believe) for the new role. So they have it when they start, and then get it renewed/updated once started.
I think that's how a contractor would do it anyway.