If you need it, have your Dr. prescribe it, which is easy and covered by health insurance, as long as you don't have other health concerns that would preclude the Dr. giving it to you. Pretty sure OTC would be more expensive than most prescription coverage's co-payments, especially if a generic is prescribed.
The health conditions that would prevent being prescribed either tadalafil or sildenafil are dangerous if present, and only a health professional would be able to determine their presence.