I got into marketing through reading about the startup world (I think some of the people on HN would be surprised at how much of it is a discipline once you get passed the BS-ers), and come more from a hacker perspective.

If you're bringing them on as a co-founder, then they're a co-founder, equity and all. It's a valuable skill as much as programming and sales are (if you're actually building a business, that is).

The issue you'll find is that most of the places to find them will be full of aforementioned BS-ers, or self-titled growth hackers. Have a small LinkedIn hunt at marketers who work in-house at a semi-large organisation - in a lot of places there's a lot of hunger to do more, but large orgs tie your hands and often see marketing as a "support" function over and above a business driving one.

It might also be easier to find a part timer who doesn't want to give up the above security, but wants to exercise their creative muscles, depending on what stage/size biz you've got and your reqs.

No idea how many are out there, but there are quite a few I'm sure. I was looking for years to partner with someone and found a couple nibbles, but ultimately no one was as serious as myself about being an entrepreneur. Let's be honest most are talkers.