My app falls into the rather broad category of "gambling, gaming, and adult". This means all US providers (I've seen) aren't interested due to their gambling regulations, and UK providers consider it high risk.

So what do your competitors do? I know it's an obvious question... but it's probably also the answer to your problem.

Pay more money than I'm probably willing right now... :)

Seriously though, our competitors seem to do it themselves. Which probably means PCI compliance and a merchant account, then doing all the integration themselves.