Dear Friend,
You are totally right, websites like Fiverr, Upwork and Freelancer are mainly good for finding a Designer or Developer for your idea but in order to find marketer you need to stick mainly to marketing community. But before that here are some friendly advice:

- Most efficient group of marketers are influencers and what attract them is some extra benefits like special discounts and.... you should mainly try to make them feel their are special

- providing good set of tools and materials like (landing pages, banner ads, video ads and...) have a lot of effect on attracting experts cause it makes they work easier and faster

- Many marketers stick to high quality product so always stick to that to get better marketers

Here are some communities which are good for finding marketers:

https://stmforum.com/

https://affplaybook.com/

https://www.affiliatefix.com/

https://www.affilorama.com/forum/

https://www.warriorforum.com/

https://forums.digitalpoint.com/

https://www.wickedfire.com/

https://www.wealthyaffiliate.com/

http://www.cpaelites.com/

http://www.v7n.com/forums/

if you have any questions or helps please do not hesitate to contact me :)

I know a lot of affiliate marketers - and am one myself. There are numerous affiliate conferences where you could find people to help. What exactly do you need?
Are you looking to set up an affiliate program on one of the platforms like CJ or ShareaSale?
You can email me at aussiewebmaster@gmail.com to discuss.

You can submit your product to Clickbank or Rakuten Affiliate networks. Affiliate marketers use these networks to find products to promote. By submitting your product to the network you will have affiliate marketers promoting your product without having to seek them out.

You might consider replacing the word freelancer with the words independent contractor. Also make your offer interesting to experienced, results-driven professionals.