Taking out a life insurance policy on someone else, even a parent, requires the insured individual's consent. Therefore, you cannot buy such insurance without your mother's approval. You can, however, pay the premiums and help complete the paperwork.
To be designated a beneficiary in a life insurance policy, you must have an "insurable interest." In other words, you must have a strong enough tie to the insured individual that you stand to suffer financially in case of her death. This is easy to demonstrate if the insured person is a parent, as children must often pay for funeral expenses. Your mother can purchase a life insurance policy and designate you the beneficiary without difficulty.
The right to designate a beneficiary on the life insurance policy rests solely with the insured person. Even if you pay the insurance premiums, your mother must sign the papers that declare you as the payee of the policy. Furthermore, most policies also require a health exam. For these reasons, you cannot buy life insurance on your mother without her agreement and cooperation.
If your mother is willing to declare you the beneficiary on the policy, you can furnish all the paperwork for her to sign and pay the insurance premiums. You can buy a life insurance policy on your mother, but only with her approval and authorization.