You may have a tough time finding a marketer who's willing to work for commission, so I'd recommend giving some consideration to your overall budget for this project before you begin your search.

That said, some places to look for marketing help:
- GrowthGeeks.com (best if you have a specific marketing task that needs doing)
- Elance.com or odesk.com (popular freelance websites - you'll likely have to do some digging to find freelancers with a lot of experience - don't just go for the cheap options)
- Ask for recommendations from friends/coworkers on Twitter, LinkedIn, etc. ("Can anyone recommend a good contract marketer?")
- There are plenty of marketing agencies out there but this option will likely require a hefty budget.

A good fit for this job is someone who:
- Has experience with a lot of (ideally all) online marketing channels. Ask about projects they've done that have focused on: email marketing, social media, content creation, etc.
- Can recommend some initial ideas right off the bat. Ask them what would be the first few tactics they'd implement if they were tasked with marketing your product, and why.
- Understands metrics and the important role they play in a good marketing strategy. Ask what metrics they would measure and what goals they would set for each.
- Has had success with other clients. Ask to see examples of their work with other clients.

Hope this helps - let me know if you'd like to discuss any further. Best of luck!