Some HO buildings can 'do' for 1/72-1/76 dioramas. Older houses and buildings, particularly from Europe, would be a good start. People used to be smaller so doors were smaller too. It would not look too strange to combine those buildings with 20th Century armor. I would think more modern buildings would be a no-go.
The alternative is to make your own. You can use railway hobby shop stuff, such as plastic sheets of 'corrugated metal', of 'slates', trees, lamp posts, fences,... I use a product called foam board to create walls. The foam is sandwished between 2 thick sheets of paper. Once you remove the papers, you can 'engrave' the foam with a toothpick or else. Then paint with acrylics.