Supply Scripts

I don't believe planets can produce supplies on their own. You need a facility to produce them.

Planets don't normally produce supplies or ordnance; you will want to search Facilities.txt for the abilities "Supply Generation" and "Ordnance Generation" to see all of the facilities that can produce it (normally only the Resupply Depot, but this can vary in mods), and then modify the "Amount 1 Formula" as needed. Also take note of "Remote Supply Distribution" and "Remote Ordnance Distribution".

If you want it to be easier for planets to produce some supply and ordnance, you could create a new facility by copying the Resupply Depot, called say "Basic Resupply", adjust the base cost way down and remove the level scaling, and give it a small flat generation ability. You'd probably also want to turn up the level scaling on the standard Resupply Depot. Editing the AI to properly use this is a more complex subject. If you want to push it a bit further, you could make it a 1 tonnage facility, and increase all planet facility tonnage by 1. (Depending on how smart the AI is, there's a chance it will eventually be added to every planet, once it's the only thing left that can fit.)

As far as I know, there's not an easy way to give planets or empires supply or ordnance without a facility, or possibly some advanced scripting. Empires can be given minerals, organics, radioactives, and base storage for the above, but that's much easier because they are empire-wide numbers, you don't have to worry about exactly where they get put.