Bing, Google ads, TrafficVance, Lead Impact etc. I would also suggest you try retargeting ad companies. And on top of that email campaigns.
But as said this is only if you wish to go big since it can cost quite a lot.
Google ads might be a problem. I tried to advertise site in my sig, but they refused the ads and asked me to send them a gambling licence...which I don't have because I'm just a referral. If OP had this licence, Google might approve it, but not in all countries...
Depends on how you cater it, there's a way around everything. And the policy is different depending on the country you advertise to, some countries are fine and do not really require anything.