Is it permissible to buy a permissible product from a company that sells impermissible things, and is that considered assisting it in the impermissible, or is it a permissible sale and purchase that has no bearing on the company's impermissible activities?

If the institution is known for selling forbidden things, then do not buy anything from them, even if it is permissible, because that would be assisting them in sin. Allah says: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression). This is if the item is available elsewhere or if there is no urgent need for it. Otherwise, it is permissible to buy from them.