This can be done at any time in the game when you have a Lander. Go to the Earth Starbase and select Outfit Starship. Select the Lander and start selling. When you have no more Landers the blank space where the Lander was will still be selected so keep selling. The number of Landers will switch over to 255 and the top of the screen will be filled with Landers. Keep selling until you have all the ResUnits you can handle.

Note: This code will only work on the original Star Control 2. If you have installed the patch, this trick will no longer work. It is a glitch and was corrected in the patch.