You need iron, nickel and sulphur for a basic reload, in low quantities.
The most efficient way to fill those up is probably laser mining, since it'll happen automatically and fairly quickly while you're doing that, but only if you like laser mining. One laser mining session should get you enough of those materials for quite a few combat sessions (and some G2 and G3 materials to trade, too) as well as plenty of money as a side-effect.
If you don't, then the only other way to get raw materials is the SRV. Just land on a planet (probably any planet will do), drive around shooting the rocks, then trade everything you get down to those materials. It'll only take a few G4 materials to trade to fill up your G1 reserves entirely.
You could go to the crystal shards worlds to get G4 materials faster, but that's more travel time and for the relatively low amount of materials needed probably the travel time is enough to make it less efficient. If you went out there and stocked up to full on G4, so that you could just downtrade to more G1 when you needed it, that would be more efficient, but also have more of an upfront cost, and it's only more efficient if you expect to do thousands of synthesis reloads.