Galaxy MapModulesOutfittingStarportsWeapons
This is the best place and not only because you will get 10% discount for all ships and modules there but also because all kind of modules and weapons are available there, however, you need to have Founders World Permit. If you are founders level backer you should have it already, but don't worry if you haven't received permit yet - anyone can get it - all you need to do is rank up to Elite (only one Elite is required and doesn't matter if it's in Trading, Combat or Exploration).
Trading obviously is smartest way because Trading Elite will net you more than 1 billion, exploration Elite will net you 150 million (from build 1.2) and combat Elite will net you nobody knows how much because it varies greatly but way below 1 billion.
So if you are trading elite you can afford any ship with best modules.
Not always but almost always you will find what is needed in first or second High Tech system visited.
You shoud:
this is high tech system with largest population nearby.
Before you go just check if stations there are not far from star by opening destination System Map and make sure that target station has Outfitting service because not all stations do have one and you don't want to waste your time - so just check what services are provided at destination before you hyper jump there.
If there are more than one station with outfitting section you can visit them because they have different modules and weapons in stock, also remember that stock modules and weapons do change slightly over time, so what you had there before might not be there now. Usually it is only temporary and module will be in stock later.
In 1.3 with power play we are promised that some powers will have specific modules and weapons to them, but for now all modules in outfitting can be found everywhere - in Federation, Empire or Alliance stations.