I've been thinking about trying to write an article on how to do this very thing.
And I haven't gotten anywhere.
But I'm about to have like 13 hours in the air and that's usually a good time for me to do it.
That said, I've actually had some decent luck using Google Streetview and this wonderful tool:
https://svd360.istreetview.com/That's how I did this backdrop.
My current approach is to generally start with a nice "generic" background that I've shot myself. These are the TOUGH part because you want them in high resolution AND there are somewhat specific requirements. You need to shoot your own panoramic photo, but you ALSO need that photo to be of a subject at a significant distance so that the angle that you're perceiving viewing things at is nice and shallow. The background here is shot doing that.
Then I'm layering on individual elements from streetview or from other photos.
Here's my current in-progress pano for behind "Downtown York" here.
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I want to say "Oh, this isn't that hard" but the more I try explaining it the more I realize that yeah, it kinda is. lol