Historically if you want to pay _lots_ of super cheap traffic, you get it from porn sites. Not the ones with monthly subscription, but the ones where each click leads you to a new domain with a 50/50 (or 95/5 depending on your "value" [US>Bulgaria, Mac>Linux, ...]) chance of landing on some scam site / ad.

Not for the companies buying the ads, you need the ask the question from them.