I'm my mother's only son, how do I go about getting her 401k?

First, you'll need to make sure that you're named as the beneficiary of the 401(k), then do her in. Seriously, 401(k) plans - and all other retirement accounts - are a contract between the participant and the financial institution. One of the terms of that contract is who gets the funds upon the participant's death. That means a will, or lack of a will, has nothing to do with the distribution.

If you are listed as the beneficiary, you can contact the financial institution that her 401(k) is with and complete their forms and provide a death certificate. If you are not listed a beneficiary and there are no other ones named, it would be payable to her estate and you would need to open a probate estate in the county in which she resided at the time of her death. I would be happy to assist you in this matter, you can contact my office.