Yes, you may be able to buy life insurance eon your ex-husband.

If you have an "Insurable Interest" you may purchase life insurance on your ex-husband. 

That would probably mean that he is responsible for child support or alimony payments to you based on your divorce agreement. 

While buying life insurance on another person requires his cooperation that cooperation can be ordered by the court. 

You should be the life owner, payer and beneficiary of the life insurance policy. That way there is never a question of the life insurance policy having lapsed or the beneficiary being changed without your knowledge.

The point of buying life insurance on your ex husband is to make sure you receive the death benefit in case he dies. You as the oner can make sure the beneficiary never changes, and that you are the beneficiary of the life insurance policy.

Here's how you may buy life insurance online.