I believe "at" is better to refer to shops: http://dictionary.cambridge.org/grammar/british-grammar/at-on-and-in-place.
Apparently she cannot buy bread IN a bakery either. This is apparently not all that odd. I just had a conversation where someone on the street wanted to know where to find Doves and buffalo....I was so sure I was misunderstanding I acted out Birds and Bovines...and the woman insisted that yes, she wanted Palomas e bufala. So maybe you cant buy bread in a bakery.