What do you get your mom for mother's day when she is dieing in a hospital from terminal cancer?

Your presence. Your presence, your touch. Look at picture albums together, sing her favorite songs or bring in music, tell her your favorite memories growing up, decorate her room. My heart goes out to you.
Yourself. I'm sure your mother would like nothing more than to spend time with you. You are the most precious gift of all. Spend time with her, reassure her, love her. Talk about things you have done together, and tell her what she has taught you that you will carry throughout your life. Ask her how you can help her most during this time. And please take care of yourself! you need love and support too!
Your advocacy. Does she have to stay in the hospital? Where would she like to spend her time? Help your mom review the practical benefits/risks/downsides of the treatments being given in the hospital. Maximize her choices. Seek to build family consensus. Get the physician's number and review the plan daily to see what the options are.
LOVE. Be there for her. Bring others to provide for a positive environment. Celebrate moments in the past with photos, items from her/your home. Consider talking to the hospital chaplain for more ideas (regardless of faith orientation).