Another way to find diplomatic missions is to go to your duty officer screen, find the tab for "department heads" and then click the assignments your first officer offers. Those are typically diplomatic, IIRC.

You can also access these by visiting your bridge from the triangle-shaped drop down menu on your mini-map. Once on your bridge, go to your ready room and check the terminal on your desk.

Also, try Alpha Triangu...whatever that sector is. It's full of Bajoran and Cardassian diplomat missions for me, usually. YMMV.