The PM part of Tylenol PM is just diphenhydramine, aka benadryl. You can get that pretty cheaply in generic form. That said, I would really not recommend long-term use; you can get resistant to lower doses, then keep upping it, which isn’t good for you over the long term. I definitely wouldn’t take Tylenol PM long-term, as acetaminophen/paracetamol (Tylenol) is damaging to the liver when taken for a long time.