Is this legal if he is willingly signs and has it notarized?
Or does he have to be the one to actually buy the life insurance policy and name me as beneficiary? He has remarried. I would pay for the life insurance policy, is this legal?
Yes, this is legal, since you have an insurable interest in your ex-husband. Since you rely on the alimony he pays you to survive, you do have a financial interest in him, and you should be able to buy life insurance on your ex-husband.
However, you will need his permission and he will have to sign the application for life insurance. Also, he may need to answer some health questions and maybe take a physical exam.
If he is okay with the arrangement, you would want to be the owner of the life insurance policy and name yourself as the beneficiary. You would be the one paying the premiums on the life insurance policy.
Here's an article that provides more information on who you can insure for life insurance?