A lot of the advice is the same as for finding tech cofounders

- People with aptitude for business development and marketing and an interest in found tech startups attend the same meetings and hang around in similar forums to developers with similar aspirations, and quite possibly overlap with your personal network.

- Personal rapport, the ability to learn quickly and ideas about how to take your product to the next level matter more than years of experience or lists of accomplishments, and so your smart friend might actually be a better fit than someone with apparently good credentials

- Potentially good cofounders will want their fair share of vested equity, and/or cash compensation

That said, sales and marketing aptitude isn't as readily tested as software development competence, so some more specific ones on how to find the right sales and marketing cofounder below:

- If you're looking for a single cofounder, then whether you need someone specialising more in sales or more in marketing (obviously they overlap, but a good specialist is like to 10x engineer) depends to a large degree on the revenue you're looking to generate per client. Your cofounder using their time to sell by telephone and email costs more per sale than your cofounder optimising website clickthroughs, but products with a certain cost or complexity will struggle to sell without the human touch (services costing upwards of, say $2000 per annum will tend to yield more with human input into the sales process even after factoring in the cost of that person's time)

Sales

- pretty much any salesperson will have an story to tell about how they were "430% over target in Q4", "hit quota every month", "brought in $1 million of new business"
Ignore this: you're cofounding a business and don't have targets, and you don't understand their targets or how tough they were. Pay far more attention to what they say about how they approached it, and how they see that as similar and different to what they would do working for you.

- Experience at Google or Facebook might mark someone out as a high-calibre engineer, but BigCorp badges don't mark them out as a high-calibre salesperson; if anything it says "this person's sales accomplishments at that particular company may have relied upon established reputation, relationships and process" which is the exact opposite of what you're selecting for. Big name clients in a vertical you're targeting might mean a little more, but only if the candidate can offer a plausible description of how their contacts or understanding of the company's internals are relevant to your product.

- good salespeople rely far more on being able to quantify benefits to an individual customer than charm or persistence, so don't be impressed with Mr Slick or rule out the guy who's geekier than most developers

- a surprising number of high-earning salespeople are sufficiently dissatisfied with their jobs to switch to the "right" opportunity without the same short term reward.

Marketing

- Marketing is a broad term covering a wide range of activities. Several years' experience in managing display advertising budgets for a big corp and ensuring subcontractors' work meets brand guidelines might sound impressive but is almost worthless when it comes to assessing whether they can minimise your cost per conversion.

- depending on what you expect them to spend most of their time doing, your marketing cofounder might actually be a developer with a passion for a/b testing or front end creative stuff.

Edit: I suppose it would be remiss of me not to mention that I fall into the "would consider right cofounding opportunity: sales experience" category