I get tons of miniatures from
Miniature Market. They single figures in addition to booster packs for a lot of different games. For a sci-fi/space campaign
Star Wars minis would work pretty well. You probably would want to avoid signature figures with lightsabers but there's a lot of very general purpose figures in the various sets. If you're looking for some sci-fi army dudes Rackham's
AT-43 miniatures are awesome.
If you don't mind a little work of rebasing the figures
Heroclix has some pretty nice looking miniatures. You may have to hunt around for some "average joe" types but plenty of the different sets have them. Like Heroclix the
Haloclix miniatures have a sci-fi feel to them.
Horrorclix also has some cool miniatures in the various sets.
I love the Miniature Market site because I can get a dozen minis for under $20 buying them individually. Most of the ones I want are "average Joe" characters rather than signature ones for whichever game they're originally from so they're often less than a dollar each. I don't play (and can't stand) the *Clix games so I have no qualms ripping the minis off their Clix bases and mounting them on conventional 1" bases. The standard Clix base is far too large to use on the common 1" square or hex maps and crowding is a major problem.