Mom is near the end of her life and weighs only 102 lbs. I can't find diapers for her 20 inch waist. Does anyone know who makes these?

If she is in bed all the time it might be easier and more comfortable for her not to have any brief on at all. A good bed pad and another but disposable under pad on top of that.
Easy to change a top sheet.
And with no brief on there is less bunching of materials that can cause irritation to the skin.
I know it may sound gross to many but this might be more comfortable for mom

Buy the largest Baby pullups/diapers you can find.

Size 4, 5 or 6 X-Large 18 - 25 in. Waist

I don't know the answer to your question, but if you mom is near the end of her life, might I assume that you now have Hospice involved? If so, they should be able to get her the proper size diapers at no charge to you, along with anything else she might need.