Small investors to use money for marketing at new startup?

I'll try to be short: I am working on a new P2P marketlpace for a niche. I think it has some future. It won't become a unicorn but it may become a nice startup / company (and it could grow, though, into other niches that share some characteristics).

Anyway, I started to building it with a friend, both of us are part of this community and we do think this would be really useful.

However, we do think we need to invest some money on marketing, before launching (to attract people to answer our surveys with their feedback), and after launching (if not the marketplace will be empty).

Are out there investors that invset money only to be used in marketing for new startups, in exchange of %? We don't want $ for salaries, or for other type of expenses (servers, etc). We just want it for marketing, because we are confident that if we reach the whole community, we can become a really important hub.