I love the plastic d&d and pathfinder miniatures. No assembling, no painting, lightweight, cheap(ish) and most importantly you can throw them in box together without worrying about them chipping or breaking.
While d&d minis have me sorted for almost every fantasy game, I’m at a bit of a loss for Sci-fi games.
Is there a similar style line that I could use for sci-fi games such as Rogue Trader, Traveller, etc?
A number of the Heroscape minis are sci-fi in nature, although they're probably pretty expensive at this point.
WotC also had a line of Star Wars minis, if you don't mind the recognizable IP. It's also out of production but much more recently so you should have better luck. Same for HaloClix.
Those are the only ones I'm aware of, but a quick Google search shows there are plenty of other option. Sites selling models they've painted, other war games (AT-43 and Mutant Chronicles came up several times).
Oxinabox brings up a good point about used models (that the previous owner already painted). You may also want to try checking out eBay for someone selling off their army.
My FLGS has secondhand prepainted minitures for cheap. They have a variety of miniatures that people have painted then presumably sold/given to the store owner, after being done with them.
Because once painted miniatures value is massively reduced, the prepainted ones are often a tiny fraction of the original cost.
As WH40K is the most popular wargame around (at least in my part of the world). Most of these miniatures are 40K SciFi miniatures.
Because they are second hand they can be made of anything, The vast majority are plastic -- that is what miniatures are made from these days. Some might be the older metal sort -- pewter, so if lightweight is a serious concern, weigh them in your hand.
Not nesc ideal for your use, but not bad. If you don't mind a bit of a crappy paint job, (or at least having to look through the cabinet til you find one you like) this could be great for you.
I suggest thus having a look in all the local gaming stores in your region.
