Is it permissible to buy barley grains from which alcohol was pressed, in order to feed animals whose meat is eaten? And is the price of these grains considered an aid to the owners of the factories in producing the forbidden substance?

There is no religious impediment to purchasing permissible goods from someone who sells forbidden things or commits sins, even if they are a disbeliever. The Companions and the righteous predecessors transacted with disbelievers who engaged in usury and sold alcohol, without partaking in their sin. However, it is preferable to buy from others if available.