Fortunately Bob didn't quit sculpting after he left Citadel, in fact he actually made quite a few science fiction miniatures for many others. I'm going to try to list them all here, in no particular order.
More than a decade ago the Cobalt-1 range was self-released, I believe in collaboration with West Wind or Old Glory. The range was discontinued, but fortunately picked up by Black Hat. I really love the retro style powered armour suits and scouts and rebels in the Mercenary range, but the aliens and characters are also very good.
In the meantime a superhero range was sculpted for the SuperFigs game. Most of them classic spandex heroes but some very useful characters too: aliens, robots, high tech soldiers and very useful henchmen squads. At the moment they're available from Old Glory in the USA and in Europe from Old Glury UK on request.
A fairly new company is Blue Moon Manufacturing
The Aliens and Spacemen range started out with a bit trashy 1950's sci-fi aliens and spaceships but recently a whole range of hard-sf mercenaries were added. Recalling Starship Troopers and the original Alien spacesuit designs by Moebius and Cobb and also new powered armoured marines. In Europe available through Old Glory UK, on request.
Speaking of Starship Troopers, Naismith also did the official S.T. miniatures for Mongoose games. Especially the big exosuits are a good alternative for the old Dreadnought Suits. These days the range is available from Rebel Minis, sold as generic SF now.
Last but not least, Warlord Games sells the Judge Dredd miniatures, created too by Mongoose for the old Gangs of Mega City One game.
At the time there were also plastic gangers, but they are not available at this moment.
The ganger characters and perps are great and of course the 2000AD universe was of huge influence on Rogue Trader, so it all makes sense! One of the Grizzly exosuits got recycled as the Dredd Holocaust Suit.
Naismith also did many plastics for many manufacturers. Still available from Em4 Miniatures are the Space Rangers. Originally released in the 1990's by Hobby Games as alternative 'Space Crusade' style monopose space marines.
He also did plastics and metal miniatures for short-lived wargames like Trinity Battleground and Havok.