I know of lots. Historical wargaming is mostly miniatures agnostic, and quite a few companies also apply that model to fantasy and sci-fi.
Focusing on plastic first, have you checked out the official range of plastics for Stargrave by North Star? They can be used for any Sci-fi or near future game, despite the Stargrave label. Best thing about the North Star plastics is they are designed to be compatible with each other, so it is very easy to kit bash a set of Frostgrave cultists with Stargrave troopers or crew to get cultists in space; mixing Stargrave with the gnolls is also very cool.
Who else? Wargames Atlantic have great plastic sets for historical, fantasy and sci-fi. Some are designed to be literally fantasy and sci-fi, like the giant spider set that comes with laser arms…
These should be easy enough to find pretty much anyway via the Web, and there’s usually some sellers on ebay that sell individual sprue at not too much of a mark up.
If you’re into kit bashing, there’s some plastic cowboys recently released by Great Escape Games that might be mixable with the Stargrave plastics.

(Sorry! Looking at independent miniature ranges has become a bit of an hobby in of itself for me over the last couple of years…)



