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 :)