A couple of alternatives to
GW ships for these games, for those who don't really care whether their spacecraft look "authentic Imperial", and just want to get down to playing these games for a low price:
http://www.ustoy.com/toy-space-cruisers
These are relatively hard plastic, and run at about .25 US dollars apiece. They are about 2-2 1/2" long each. They are partially painted. I'd use them as battleships / capital ships in conjunction with the other stuff listed.
http://www.ustoy.com/space-cruisers-72-pcs
Extremely cheap, coming in at about .08 US dollars apiece. These are maybe 1" long. They are a softer plastic, which will not be to everyone's taste, but for this price, I think you can afford to splurge. I'd use them as cruisers, destroyers and maybe fighters (if marked in some appropriate way).
Also, US toy does not charge shipping,at least within the US, which is always a bonus.
On the American Science and Surplus website,
http://www.sciplus.com/ check out:
"93013 GITD SOLAR SYSTEM"
This is a nice solar system model that gives you the sun plus the nine planets (Yes, Pluto IS a planet; I don't care what a bunch of fussy astronomers say). They are fully round, rather than hemispheres, but they look good. Not really in scale with the spaceships, but I always say the planets take up their full area, while the ships are only mathematical points represented by the posts. Get two sets and you can even do a binary system.
Also from that site search for: "Buck Rogers!!"
27785 BUCK ROGERS FIGHTER PLANES
27786 BUCK ROGERS GUN PODS
30331 BUCK ROGERS SPACE SHUTTLES
30332 BUCK ROGERS
WAR BIRDS
These are about the size of the smaller ships from US toy, and can be used for similar purposes. They are more expensive, but made from hard plastic, which many people will find more palatable
Anyway, for about thirty bucks you could get all the ships and planets you would ever need to play these old games. If you want to play spacefleet (which was far more fun) all you have to do is make your own drop lid (I'd use a square metal cookie tin, painted black and marked hit and miss appropriately on the inside) and you are ready to go!