This event never happened in the books so your books tag is meaningless in this regard. As for the show, As of now, we do not know where did she get that face. Right now it would be just speculation.
There are following possibilities:
- She killed some woman and took her face
- She stole it from House of Black and White
- As suggested by Kl78, She may have found a corpse and carved its face off. She may not have killed her after all.
Did she kill some person?
It is a possibility but it is also unlikely. Arya has only killed people who were either on her hit-list or were trying to kill her. She refused to kill lady Crane even though she knew that would mean she failed her tests. Therefore it is unlikely she killed some innocent person.
Did she steal it?
This one is more likely than the other. Arya killed the Waif and put her face in the House of Black and white. Jaqen only entered after Arya had finished putting the face on the wall therefore we do not know if she actually did what I believe she did.
Did she find it?
It is plausible but less likely. The odds of finding corpse of a good looking young woman just lying out there on the road are very slim. She needed face of a such a woman to find entrance into the castle in disguise of a serving woman.
Lord Walder Frey did not recognize her so that might mean she must have been let in that day only due to her looks and because help would be needed in kitchens to feed the Lannister guests. (Although Lord Walder’s advanced age must be taken into account here, especially how he can’t tell his grandchildren apart sometimes or mothers of his children).
Good looking people get away with things average and bad looking people don’t usually do. Finding entry into a heavily guarded castle hosting very important people is one of those things.
Do we know her?
No we do not know whose face it was or if it is in any way important to the story.