Consider using networks like Google AdSense, Media.net, or Adsterra for direct link ads. You can also explore affiliate networks, such as Amazon Associates or ShareASale, for relevant advertising opportunities. Be sure to review their requirements and guidelines before signing up.
I use AdMaven, which works quite well and is giving me high CPM rates for all traffic, both for direct links and smart links. If you also use their content locker, you can increase your income considerably.