Not sure if these guys still have stock: https://shop.prodosgames.com/8-alien-vs-predator
Imperial Guard...sorry, Astra Militarum from Games Workshop have some cheap marines types, obviously the weapons are not exact as it's a different franchise: https://bit.ly/2NOCzJ7
(That said if you've got a pair of clippers or a hobby knife if you snip off the barrels of their weapons you basically have a pulse rifle)
And they have Tyranids which are HEAVILY inspired by HR Gigers work: https://bit.ly/2JXJQVQ
Mantic have some models that would be good for a Weyland Yutani Commando purge team or similar: https://www.manticgames.com/games/warpath/page/2/
All the Games Workshop stuff is 28mm scale thereabouts, not sure about the Prodos stuff.