What is the best mattress to get my grandmother for her hospital bed?

maybe you could call a hospice company, see IF they could recommend something (NOT saying you need their full services)

or maybe a hospital supply company.

There are certain mattresses made for hospital beds. They have to be able to bend so interspring maybe out.

If she has an adjustable hospital bed frame, it's only important to get a mattress that fits the bed and will bend as the frame moves. No interspring. Also, if she spends long hours in bed, she may need an alternating pressure mattress topper.

You want to think long term and get something that is formulated to prevent pressure sores, so I think hospital mattress vs regular adjustable (sorry I don't have a brand, you'll have to do a little research). And buy the very best you can afford, it will be cheaper in the long run than having to buy 2 because you need to replace it later.