If your gona do the supplies manualy keep the mats stocked in your country and when your corp needs something sell it to them in direct trade. But like crafty said good luck keepin them all stocked, if u have alot of corps it will get very tedious very fast but if u got lots of free time to micromanage your corps this might help u keep the costs down in your corps.
If you have a lot of corps, you can contract supplies to each other. I do this in my CEO where I contract low-level commodities (like sand, paper, and such) at high quality so that I can buy high level commodities (like FMUs) at 100 quality. It is an overall cost cutter but tedious while you are setting it up.