Looking for a mattress?

I am looking to buy a Comfortable mattress bed for my mom, she have a back pain problem, so I want to buy for her to give a comfort and support, firmness which is she needed, and one of my friend bought mattress through Luma Sleep coupons last month, and he's suggest me that I buy from there, but I am afraid to buy from online because I have not bought online before. Anyone here who can share her online experience?