Lots of skins here, probably some other stuff too but the navigation is time-consuming (it goes by creator, not category I HATE that!):
http://ll30sims.com/#/startrek/4529154168Simslice has a Guardian of Forever and a bunch of other stuff. Pay site but they have a cheap one-month subscription option that's totally worth it.
DeviantSims has a lot of SF/Alien themed skins, walls, floors, and objects