Piracy is rampant for adult-rated films because theaters are reluctant to screen them (fewer advertising sponsors). However, for viewers wanting to support the industry, here are legal sources:
While partially Italian, Machan features a Sinhala cast and language. It was rated 18 for its depiction of poverty-driven desperation, including a scene where characters consider self-mutilation. It is less violent but psychologically intense. sinhala 18 movies