How to find designers who are also well versed in marketing principles?

It's near impossible to find a designer who has deep knowledge of what converts and what doesn't.

It seems you can either find someone who can make something really good looking, but lags in conversion, or someone who can make something that converts well but looks horrible.

How do startups find designers that can do both?

The easiest way is to poach them from companies that have great design / UX.

Talented designers have no shortage of opportunities, so they are not likely to be looking at your job listing. You need to actively reach out to them.

They are so few and far between, that most startups won't be able to get one.

What now?

How about a great designer and a marketeer working together with a mindset of "growth hacking" - that's much more easy to find, and probably not much more expensive than getting the unicorn you're searching for.