Also, some ships are only available with Horizons, so if you can't find a particular ship that is supposed to be at a particular station and you don't have Horizons, that's why.
On a final note, even if you do have Horizons you will never get the Cobra MKIV.
When you rank up in the pilots federation you may gain access to Jameson Memorial station in the Shinrarta Dezhra system. There all ships and modules will be available to you.
You can also get discount in systems run by Li Yong-Rui.