OP, my daughter is 12. She doesn't actually need a bra but I know all her friends are wearing them. I also know she would not want to go bra shoppingn due to embarrassment. I just went to Target and bought some little bralettes (3 pack) of Hanes or whatever in cute colors and said if she didn't like them or they didn't fit I would take them back. She huffed and puffed and took them to her room but she was wearing one the next day and every day since
My point is, it might be too embarrassing for her to do with you, with mom, or even a friend's mom. Take a stab at it based on her clothing size.
I'm assuming when you are talking about fit, you mean you want her to get something that is the right size and not inappropriate. I don't read anything weird into your query and think you are kind to work on this issue.