Are contractor's liability insurance policies only written if the policy holder also gets workmans' comp insurance?

Contractor Liability Insurance

No, You can purchase Contractors Liability Insurance as a standalone coverage.

Many companies offer Contractors Liability coverage with or without your workman's compensation coverage. Depending on the nature of your business structure, you may not even need Workers comp coverage.

It is perfectly fine for you to purchase the two coverages separately. You may however get a better rate if purchased together because some companies will offer you a discount for placing multiple policies with the same company.