I personally do not have any casino website so I can't vouch for any of these. I'm thinking that casino traffic won't come cheap as it is a very tough and competitive market. Howere, here are a few links I found that you may want to take a look at..
Online Casino Advertising Techniques to Increase Affiliate Revenue Cost per click casino advertising with WassOnline, best casino and poker traffic source Buy Website Traffic - Adult & Targeted Traffic Increases sales
Try using forums that cater to gamblers, that is probably the cheapest option.