We noticed that there's organic ones (in tree knots/clouds) or random ones (cracks in cement, rust peeling) or man-made ones (anything designed).
I especially love ones involving a shadow hitting something at just the right angle - those feel extra hard to find :)
Thanks for sharing this!
It's a bit old-web now (on flickr! ancient history!) but they have a lot of good ones - https://www.flickr.com/groups/facesinplaces/pool/
And even put out a book a while back.
I like this horrified house - https://www.flickr.com/photos/chelsea_steve/55448046101/in/p...