What is the ruling on eating food products (such as bread and milk) that are suspected of being mixed with unknown animal substances, if no alternative is available or if it is more expensive?

The general principle regarding food is that it is permissible, unless it is mixed with impure or unlawful substances. If it is mixed with something unlawful to eat, such as pork fat, then it is not permissible to eat or purchase it. However, regarding what is mixed with a small amount of carrion that would benefit from ritual slaughter ( dhakat), the preponderant opinion is that it is permissible to eat, especially if it is difficult to avoid, and a small amount of impurity in a large quantity of food is overlooked. The Companions ate cheese from the Majus (Zoroastrians) despite knowing that rennet from a calf was mixed with it, and that their slaughtered animals were carrion. Moreover, what Allah has made lawful is halal, and what He has made unlawful is haram, and what He has remained silent about is that which He has pardoned. Based on this, there is no impediment to using these things, due to the lack of certainty regarding their impurity, because food is not discarded due to doubt, even though refraining from them out of scrupulousness is more cautious.