Just go into a Federation controlled system, scan the nav beacon, jump into supercruise, and check your nav panel for High Grade Emissions. Fly towards it. If it has a state of "None" it's highly likely to contain both proprietary (g4) and core dynamics composites (g5).
It's super easy, and after finding just a few High Grade Emissions zones you should have more than enough for engineering. And trust me, you'll want those core dynamics composites later on for engineering your armor/hull. Best to stock up while you can.
It's better to cut out the middle man (material trader) whenever possible in this game. They're worse than used car salesmen.