Besides Star Wars minis, you can try some of the varous WizKidz Clix lines. Ask your FLGS proprietor or someone else who might know- some sets are more sci-fi-ey than others. For example, the
Armor Wars set features a LOT of characters who wear power armor (Iron Man, Titanium Man, Crimson Dynamo, Guardians, War Machine, The Ghost, etc.)
In particular, the current
Alien vs Predator and
Halo sets are also a good buy in that regard.
I'm sure some of the MageKnight and HeroScape minis would also be appropriate for most campaigns as well.
If you need mechs of some kind, consider the
MechWarrior sets (which would really be more like power armor in scale with other mini sets), the
X-Men Sentinels clix, and Rackham's
AT-43 (
http://www.tabletopgamingnews.com/?p=5186).