Where can you get per job workmans comp insurance?

The short answer is... you probably won't be able to find it, ever. There is simply not enough premium collected from a short-term workers comp policy for an insurer to want to accept the potential cost of employee injury claims. To make this more meaningful consider this... your workers compensation policy has no specified maximum limit of coverage (don't confuse the employers liability or part II coverage) except for the settlement amounts and conditions specified by your states industrial insurance laws. The potential payouts are astronomical and can last for decades.

That being said, there are types of aggregated policies, such as OCIP's, that include workers compensation as part of the entire portfolio of coverage, for one large construction project. But the premiums for these types of policies are often in the MILLIONS.