In the modern internet environment, everyone is wary of advertising in all its forms. You can pay for ads on related websites or in search engines, but they can be hit or miss, and if you are not immediately and directly converting your visitors to cash they can be prohibitively expensive. You can try some kind of grassroots campaign on forums, but militant forum-goers are usually quick to shout down so-called "shills." One of the few sources people still trust is what they find on their favorite search engine.