Reaper has a couple that are cool:
https://www.reapermini.com/OnlineStore/casualty (Fantasy knights)
Mostly I make them, myself though. It's just easier in one sense (harder in execution maybe of course).
That's how I did these guys:
takes a bit of cutting of course, but you can do a good bit with that, and a little
GS if required too
Skeletons are a good way to start! I got a box of Tomb Kings skeleton calvary and used them for skelly horses/riders. The horses were great because they came in two pieces so you could make the main part lying on the ground, looking very dead.
anyway, Black Powder has some casualties, Hail Caesar as well. I don't know of any Sci Fi ones except the
GW SM casualties, but you might check miniaturemarket, wayland etc
Good luck!