I have found Udimi to be well worthwhile. There a many seller there and the prices are affordable from as little as .23cents per ad depending on the amount of people you want to present your opportunity to. There is also a forum on site where you can ask any questions you may have.
On top of this, if you join as an affiliate, you can even earn some residual income to infinity when other people use your link to join and buy advertising
Herewith a link to the site: https://udimi.com/a/sl57f ( if you decide to buy advertising using this link, I will be earning a commission)