I need FAA approved portable oxygen for my mother. Does anyone have any recommendations?

My mom uses Inogen. It’s the only FAA approved concentrator she has used. The only downside is that you can’t just go to the medical supply company when you need supplies or something fails. (When she used Lincare, she could always go down there & get what she needs if they couldn’t send someone right away). With inogen, when she needs supplies or a battery charger goes bad, they have to ship it to you. It sometimes take 2 days which is a long time when you are on oxygen. But other than that, she really has no complaints.