You want to advertise your site on other sites. You might buy those online avails on other sites, or hopefully be able to save some money and barter ads between your site and theirs. One of the best-kept advertising secrets on the Internet is advertising on email mailing lists. There are over 60,000 of them. Now many of them don't accept ads, but lots of them do.
Ads are cheap to buy and cost nothing to produce because it's all done in email.
Make sure you advertise on a moderated list where the moderator keeps the discussions on-topic. Ultimately, whether you advertise with banners on sites or on lists, make sure you give your target group a darn good reason to come to your website. You must must must consider offline promotion of your website as integral to your plan to attract traffic to your site. Maybe your content or attraction is so good it gets talked about and pointed to in the trade press. Nothing builds traffic like a third-party editorial endorsement.