For Player Characters , you can use the Space Station Range by Ground Zero Games/Eureka . They are modelled after EF officers , and have a good quality . I have both a male and a Female Officers (there are 2 variants of both) and I am currently painting them as EF fleet officers , perhaps I could post pics of them on a friend's homepage in a week or two .
As per other characters , both GZG/Eureka and Denizen miniatures have a wide range of good quality SF troopers , characters , creatures and equipment that can be useful for RPG . I am planning on paint several Denizen space adventurers in a Firefly-sort of fashion when I'm finished with the EF officers .
I also have a few Harlequin B5 minis . They are of 28mm scale , and not really compatible with the previous companies 25mm figures . They have good quality (for their price) , but unfortunatelly they are no longer produced or avaiable .
Ground Zero Games : http://www.gtns.net/gzg/
Eureka Miniatures : http://www.eurekamin.com.au/
Denizen Miniatures : http://www.denizenminiatures.co.uk/Main.htm