There are quite a few casinos out there and most will offer a referral program, you'll probably have to get individually approved. Stake.com is fairly straight forward and I think you can get going with the referral program almost immediately without any further verification. If you check out the betonline.ag site their referral program actually covers a bunch of different offerings so might be useful if you want to promote multiple different sites from one brand. Besides them, just look at any of the threads here which are hundreds of pages long and you'll probably be in safe hands with the affiliate program there. The exception being 1xbit, because they are total scammers.