There are lots of good ranges of miniatures out there which can be pressed into service. Anything from "other" Science Fiction ranges, to WWI, or even American Civil War can be pressed into use. With a tiny bit of conversion and the right paint job, even earlier ranges can be used...
_Writing_ miniatures rules is no big deal either: you find a few sets you like, and steal bits! The first couple of editions of Striker are a good start. Other GDW rules sets are another good source, since Frank Chadwick, in particular, has a distinct style that carries across from one set to another.
Most WWII, or better yet, post WWII rules can be converted into SF rules without too much drama. Artillery is artillery, tanks are tanks, and grav vehicles are basically helicopters with armour!
Alan Bradley