פירוש על עבודה זרה 70:19

Two amoraim note that even if Rabbi was looking for a Gentile baker, this does not mean that a Gentile baker could always be used. First of all, he would only allow a Gentile baker if no Jewish baker could be found. In other words, ideally one should eat Jewish bread. Only if there is no choice would one be allowed to eat Gentile bread.
Two, this would only apply in non-social situations (in the field). In the city, this would always be prohibited, to prevent fraternization.