All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Essentially, it is permissible to purchase all permissible food items from such stores, regardless if they sell alcohol or not. However, since these stores that sell alcohol are in Muslim lands, it is better to avoid buying from them if there are other alternatives, unless you are in need of something only found at these specific stores or it is difficult to find elsewhere. This is because boycotting such stores is a way to show your disapproval to their owners, and the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever sees an evil must change it with his hand. If he is unable, then with his tongue. If he is unable, then with his heart; and that is the weakest of Eemaan (faith).” [Muslim]
It is known that if the Muslims shunned these shops, enough financial damage would incur in order to deter the selling of wines and other forbidden things; so they would be forced to respect the sentiments of Muslims and they would be prevented from publicly selling vile things among the people.
However, the boycott by one person or a few persons has no effect. Rather, the people must help one another and come together and make their voices heard and thus their boycott of those stores will be effective.
Allaah Knows best.