I did manage to acquire quite a few sci-fi minis, but they're all metal, I'm afraid. You can get pre-painted plastic minis for D&D by the truckload but I've never seen sci-fi ones. Having said that, I never went out of my way to look for them.
Metal ones can definitely be obtained but aren't all that cheap. Most modern ones are pewter now, although I have plenty of lead ones (worth also noting that I got most of mine in the early 1990s).
I've got some official Traveller (and Space: 1889) ones, Denizens, Star Wars, Shadowrun, cyberpunk, aliens, starship troopers, various misc. Ral Partha ones and a handful of others. There are quite a few outfits selling them on the interwebs; a little google-fu should readily turn up result.
As for plastic ones, I can really only suggest web searches. There might be some out there. A little searching picks up a game called Star Wars Legion that has minis. It's not specific about whether they are plastic or metal, but says not suitable under 36 months - that suggests the minis are probably plastic. A little more google-fu turns up stuff that looks like it's probably plastic, so I guess there must be at least some out there.