Laser cut MDF is going to be 97% of what's available, I imagine, but you don't want that... Try Etsy? There are a lot of people there selling 3D printed stuff, so you can get 3D printed items without the printer. You pay a premium, of course, but it may get you what you're looking for. Ebay may have similar. Also, if you're buying from a 3D printer, ask them about resizing models. For example, if you're interested in buying pipes, ask them to make them twice the size for some big pipes to run through our layout.
Another option would be to stick with the Sector Mech sets, but customize them as you add duplicate pieces to break up the duplicicity. The new Kill Team Space Hulk set has a lot of little greeblie pieces for adding on that would look good anywhere. The Dollar Tree stores have a line of action figures called Final Faction that have accessory kits which make good greeblies too. And the Ash Wastes terrain is compatible with Sector Mechanicus if I recall.