Upgraded versions of Miner Rebellion ships available for purchase from the Blood Amber Shop(a.k.a. Larus's Black Market).
Using purchased credits from the shop, you can then choose to upgrade current, standard versions of certain ships, with each upgrade consuming 1 credit. These ships are distinguished from others by using yellow or gold coloration, along with the engines instead emitting a yellow flame instead of a light, sky blue one.
These credits do not allow you to obtain ships you have not researched and built yet. You must have at least one ship built to be able to apply an upgrade.
Also important to note is that these are listed as "Enhanced" ships. They will soon be differentiated from the older Mk II ships, which will gain new characteristics.