You don't have much to do to obtain a correct 1/144 diorama. First you alway have the choice to do something simple, with vegetation (some trees can be big, from HO scale railroad models), buildings (in NO scale from railroad models), factory or hangar (Kotobukiya have a bir range of kits like that), ruins...
Here I can provide some help.
I've done a diorama for my 1/144 First Grade Zaku II (char's model).
For this project I simply use some PC monitor parts I found on the ground. Some people smash the TV and monitors that were trhown up, so there is plenty of opportunities for me to gather pieces when I'm out.
This look like that...
You'll understand what you can do with it if I show you this picture...
Then what ?
I took a car circuit toy, I found on the ground too (I find a lot of things actually), completed the bridge like a broken one (the road is made with sand paper), add little rocks, wood powder and stuff and "voila".
Another simple example. I took the top of the package of one of our famous cheese, the "camembert". It's wood base, and a circle. I placed and glued some "ardoise" a rock that was used to manufacture our ancient board in classroom, add some paste made from wood powder, give it a shape of a lanscape, with footprints (Zaku footprints) and from this...
With some white primer, chestnut ink and a white drybrush you obtain that...
Hope that can open your mind to endless possibilities