Doing the exploration missions I sometimes enter systems that are called "Aid the Planet". I talk to the planet and they need, for instance, 10 shield generators. How do I complete these missions and where do I find these shield generators?
There's a vendor at Sol Space Dock (in the requisitions area) that sells those things. Buy them there, take them back to the planet. (I'd buy a surplus, so you have more for next time, if you have the EC to do so.)
Actually, the "Aid the Planet" missions never ask for anything you can't replicate directly from your ship. Just hit the "Replicator" button at the bottom of your inventory, replicate the amount requested and re-hail the planet to deliver the supplies.
Although 50% markup, it is easier to complete the mission using the replicator. Otherwise, you'll need to get to the Commodities Broker, buy what you need, then head to the mission.
Since you can hail for missions anywhere at anytime, then the Replicator is more efficiant time-wise at the cost of paying a higher price to save time ... that makes sense, right?
If you are cash-strapped, then you'll be flying a little bit longer