You can use Upwork, but the problem is that you'll receive 50-100 replies from all over the world. And it will be hard to select a subcontractor.
When choosing an agency or a single sales/marketing manager (it can be an individual freelancer) note the following:
● Decide what kind of marketing channel you plan to choose (talk to a couple of agencies and sales experts to understand what kind of activity will work in your particular case).
● Each agency (the same with freelancers) is good in some particular narrow strategy. For example, my agency works with B2B social networking (LinkedIn, Facebook, MeetUp) and email channels. We don't work with other strategies, like SEO or PPC. So, when you know which strategy works in your case, it would be easier to select a subcontractor or find a right employee;
● Don't try to recruit people for a % on a deal. Nobody wants to test your unknown business model, people would work more eager if you pay for some interim results, or quantitative work (i.e. getting a negotiation, or getting a contract signed);
● Test the agency on small amount of work, let's say, order one week and see results, or order one piece of result and see if it works. Again, it is better to spend some money and pay a reduced cost for this work. But in some cases people may agree to do it for free.