You can accept a contract for plutonium in your nuke corporation too. You can also move it through space by shuttle I think (I've never done it but I don't see why not).
The nukes themselves also cannot be sold on the world market, you must either direct sell or set up a contract.
Tip: If making your own plutonium to supply your own nuke corps, make the plutonium at 100 quality. That means supply quality 100 and no quality upgrades. Otherwise your nuke corp will be needlessly expensive.