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.