This link was shared over on another thread but it deserves it's own.
PaperKraft City sells full-color backgrounds and buildings for dioramas. They're scaled the support a few sizes, including 1:64 diecast like Hot Wheels.
I haven't purchased any (yet) but the kits look to all run under $10, usually around $5, so it's affordable assuming it comes as a nice high resolution file. The only down side is you have to print it out yourself. This isn't difficult or very expensive but you'll want some thicker paper stock to print it on and it's going to use a lot of ink. I bet he could add $3 to his prices for pre-printed kits and they'd sell...I'd buy.
If you're looking for a quick and easy way to spice up your dioramas for photos, this is might be a good one to try. You could probably also do some Googling to find some free backgrounds but your quality may vary in that regard. I'd like to think PaperKraft comes ready-to-go without worry.
Here's the link to the diorama backgrounds...many scenes to choose from
And some quick pics: