Depending on your startup, (1) just equity can certainly be attractive to the right person. (2) The kind of marketing experts you're looking for can be found by searching on google for blogs relating to growth hacking, SEO, social media, customer acquisition, UX usability, etc. (3) I havent heard of Consulting Firms that do marketing for equity. There is a big company in Germany that takes equity in startups in exchange for ATL media campaigns (on tv, radio, etc). Its a long process though, like with any other investor.

Could you please share your Beta version so that interested parties can contact you?