Tugboat,
I make a lot of structures myself from balsa, cardboard, Plastruct sheets and windows. I have a LHS (local hobby store) for lichen, grasses, trees, etc. Scenic Express has just about everything you could need. I told you about backdrops from Backdrop Junction. For building flats, all you need is some 1/4 inch black foam board to mount the pics to. I downloaded and photo printed most of mine, but, for really cool ones, you can get them online.
Woodland Scenics has the best lighted buildings, but most won't fit on a shelf layout. Trainworld and Mario's Trains have most of what's available from manufacturers.
Here are some pics for ideas:
Large backdrop, Miller Engineering signs, stick-on decals on a wall.
Lionel factory is way too big, so I made this one to fit the space. Added Miller Engineering signs, which I have a dozen of on the table layout, and seven on the shelf layout. Perfect for your overhead, easily mounted, great light sources.
Lots of ways to make mountains and hills. The rocks are simply carved and painted foam from shipping cartons, the grasses and lichen are from Scenic Express.
These building flats are plastic and I made one out of balsa. The mall above is carved 1/4 inch foam, plus storefront pics, LED lights, and left over tiles.
You could make something flat like this, with a lighted Miller Engineering marquee.
The roadside stand on the left is a Plasticsville piece, with LEDS under the roof. The Red Man factory and Burger King are flats, with mounted 5mm colored LED lamps for added light. Both are small enough for a shelf project.
One of my favorites is that Lionel Luxury Diner on the right. It's a real beauty.
Hope this gives you some ideas.....
Jerry