I make my stuff from left overs. Oil or Water Tanks, smoke stacks, etc. A lot of packaging makes for great buildings.

My go to fave is the Corrugated drain pipes. cut in half and you have a bunker a rhino can park.....a quick drybrush of metallic and its done in 5 min. Or add a base, walls, doors, vision slits, etc.

go to Thrift stores and you can find old childrens space toys to damage/modify for terrain.

good luck