Treat this just as would if you where hiring a paid employee, except that as freelancer, they can work remotely and the hours that they choose.
Develop your job description, including an overview of your company (what you do, benefits of working with you), the roles and responsibilities, the 'skill-sets' (hard skills and soft skills) you are looking for.
Specify that this is a commission based role and whether or not training is provided.
Ask for references and check those.
You will find that most people are looking for at least some sort of base pay - but you never know - I have seen digital marketing agencies that have done quite well with commission only staff.
Post on the job boards (Google "Job Boards for Freelancers" and don't forget about job boards like Craigslist, Monster, Workopolis).
You can have a 'Careers' section on your web site too, and post the job description there.
Also get the word out on social networks (business and professional) and let people know a) to tell their friends; and b) who to get in-touch with if interested.
To attract good talent on a commission structure, you will need to make the potential pay very rewarding. To keep that talent, you will need to work with your commissioned sales staff to make sure that excellent remuneration is achievable.
In particular, for commissioned sales jobs, you will be looking for professionalism, very outgoing personality - someone with a lot of energy, initiative and especially confidence.
If your commission structure is good and your services are 'sell-able,' then both you and the commissioned sales person should have an idea, after about 1 week, whether or not you have good job fit for the salesperson and a good salesperson for the job.
If they show promise, reward them and continue to help them get better at sales.
However, if after a week, it becomes obvious that they aren't showing promise, get them to move on.
Expect to have a high turnover rate for commission only sales positions - the most talented commissioned based sales people are like mercenaries - they'll go where they make they believe they can make most the most money, and why not?
So think about the payment structure and if that is what you want for your company.