How old are you? Can't you just go out and buy your own? Why do you want one? Otherwise? just ask her, see what she says, it might/might not be good, I guess you know her better than I do. If my daughter Gina asked me that I'd have a heart attack.
When I was younger, I decided I wanted one but didn't know what my mom would think. One day when we were shopping and went past the underwear section, I just mentioned that I thought one was cute. She thought wearing a thong was crazy but she said I could get it if I wanted it cause I told her I just wanted to see if it was comfortable.
If you want one to look sexy, then don't bother. If your at the age where you need to tell your mom what to buy then you don't need to be sexy.
If you want it for comfort, tell her and she might understand,
If you need it to impress your girlfriends, then just stop. That mean-girl "look at my stuff" attitude will not get you real friends. It was only get you competition and bitchiness.