Awesome question. Peter Drucker, one of the world's brightest business thinkers and the man who invented the concept of Management by Objectives famously once said, "Business only has two functions - marketing and innovation."
It's astounding to me that early stage teams so poorly value marketers. Not "marketing" mind you... marketers. Marketing is easily misunderstood, especially in our era of Lean Startup, low risk investment, and focus on "what customers want" - not that there is anything inherently wrong with those philosophies and ideas, rather, they too easily teach (inappropriately) that marketing is about acquiring customers; and therefore, that you don't hire one until you have a product, know your customers, and have figured out what to charge - so you can pay to acquire more.
This from wikipedia; unfortunately, many mis-read this or stop at the first sentence, "Marketing is the process of communicating the value of a product or service to customers, for the purpose of selling the product or service. It is a critical business function for attracting customers."
What's usually misread there? "VALUE of a product" and "Critical to attracting customers"
Marketing doesn't start when you're product is launched or after an MVP is up, it defines what your MVP should be. Marketing is the process of understanding the value of your idea in the market and, I don't think anyone would disagree, it's critical to attracting customers (something you'd want to do first before investing in development).
But it goes on... "marketing is the link between a society’s material requirements and its economic patterns of response. Marketing satisfies these needs and wants through
exchange processes and building long term relationships.... Marketing is the science of choosing target markets through market analysis and market segmentation, as well as understanding consumer buying behavior and providing superior customer value."
That's why I distinguished a marketer from marketing. You can hire anyone to do some marketing, someone with experience in Adwords or email can do some marketing for you. A marketer is the person who can ensure your product meets the needs of the market through market analysis and segmentation.
One more thought, "The set of engagements necessary for successful marketing management
includes, capturing marketing insights, connecting with customers,
building strong brands, shaping the market offerings, delivering and
communicating value, creating long-term growth, and developing marketing
strategies and plans."
So, now a thought on an actual answer. To what extent can you hire an agency to do that in a startup? A "partner." (rhetorical question, I realize "partner" doesn't necessarily mean 3rd party to you - point I'm trying to establish is that a marketer should be part of your earliest team). To what extent is pure customer acquisition the real value in marketing? Make no mistake, Peter Drucker wasn't so ignorant as to conclude marketing meant simply acquiring customers for your MVP when he said that. Business has only two functions, the marketing I've defined here, and innovation. Almost any other form of marketing will, worst case, result in your failure, or, best case, be a significant waste of money as you spend to acquire customers without really marketing.
As Duane suggests, you have to seek them out and try to lure them; that job is not easy and many
people who think they can do it cannot. After only 2-3 months many burn
out completely. Why? Most startups hire someone to DO customer acquisition and most often, at an early stage, with an incomplete product, and a limited marketing still set, most customer acquisition will fail.
I think Duane's advice to check out Klout is cautiously optimistic. Klout measures the social influence of someone and like any technology, it can be misleading. On one hand, it isn't hard for a marketer to get a high Klout score, on the other hand, you don't necessarily want/need a social media maven. Yes, it's an indicator of someone worth a look (I love seeing LinkedIn profiles for people that claim to be bloggers - and they have terrible blogs - or social media marketers - and they aren't even on Google+) but being active in social marketing is about as leading an indicator as finding someone for Business Development who has a lot of connections on LinkedIn.
And coincidentally, that's where I'd start. LinkedIn. Forget the cool new tech, HR tools, networks, etc. LinkedIn is still the tried and true network on which to vet someone's background, skills, network, etc. It's not right all the time, but there is no better. Start with a search for "marketing" in your industry or at related companies and start digging through networks to find good people. Then network network network... marketers by nature like to meet people so it isn't hard to connect with one and start making headway to find the people recommended in your industry, where you live.
How then do you attract them? The same way you hire Rockstar Developers. Great marketers want a challenge, they want to do innovative work, they want to prove they can, most importantly, they want to work with a team that knows there are no right answers - at least not if you're innovating. If you are looking for someone to acquire customers or run some marketing programs, look for a recruiter and hire someone who can run a campaign. If you want to attract and appeal to a Marketer, explain why you know you need one and they'll line up at your door.