You can find adult traffic through specialized ad networks. Compare their GEO coverage, traffic sources, minimum budgets, and targeting options before starting.
I remember my first few campaigns. I changed the creative, landing page, GEO, and traffic source at the same time. When performance changed, I had no idea why. Now I isolate variables whenever possible.
Start small. I learned this after wasting money on a large campaign before understanding the traffic source. A small test would have saved me a lot of trouble.
The offer matters too. I once assumed a traffic source was bad, but after changing the offer, the results improved considerably. Sometimes the problem isn't the traffic alone.