I feel so bad after being unable to buy my father's medicine because it is so expensive and the generic medicines were already bought by the others

If you call his doctors office, sometimes they can give you free samples that were dropped off by pharmaceutical reps.

There is also a website where you can shop for the lowest price on the RX in your area if you are in the United States:https://www.goodrx.com/

If you are in the United States state by state pharmaceutical assistance: https://www.medicareinteractive.org/get-answers/cost-saving-programs-for-people-with-medicare/state-pharmaceutical-assistance-programs-spaps/spap-basics