It is possible to use either 'at' or 'in' with places where you buy things.

Do make sure that you use articles correctly, though. Remember that a single countable noun always needs an article. Here are some examples:

Buy a goldfish at a pet shop.

Buy some drugs at a pharmacy.

Thank you very much for your comment. I am never sure about articles. There are my Achilles heel. :D

at the supermarquet

hi¡¡ I think that we buy bread in a bakery. hahha but I'm not completely sure...