The Holocron - Under Downloads --> Tiles there is a download called 101 Map-Making Objects. These are all for the Star Wars universe (but I'm sure you could apply them to other Sci-fi settings). I use the computers and chairs from this object pack quite frequently.
RPG MapShare - Their gallery has Sci-fi objects (vehicles, spacecraft, creatures, structures, weapons, etc.). I believe the tanks from the gas refinery overlay came from the "Modern" objects section of RPG MapShare.
Wizards of the Coast SWMinis Forum - There is one thread in particular (Maps and Elements) which has a lot of good objects for the Star Wars universe.
Dundjinni Modern/Sci-fi Forums - I've found some good stuff from these forums, but sometimes you get things that are specifically designed for the Dundjinni software.
NASA Picture of the Day - I use this site for background images. They usually have a Hubble picture two or three times a week.
I have made my own objects. Right now it is just a few, the reactor and the generator. I'll start up a new thread in "Mapping Elements" for those. I usually try to find it somewhere first (there isn't any reason to re-invent the wheel) and if I don't find what I'm looking for, I'll make it. So far, the objects I have created are based on fan art of new modules for the Battlestations game. I wanted objects which were present in the Battlestations tiles and couldn't find something close - so I made my own.