I sometimes find them when looking for a suitable photo for something else.
Otherwise I mostly find them by chance in thoughts, and then there are the times I want to do some but cannot think of any, and I generally look around and eventually something comes to mind!
I keep thinking, soon I will run out of questions, but it hasnt happened yet.
Two mornings I took a tape recorder when I walked the dog and I got about 12 in 30 minutes...but is a hassle to turn it on and off all the time with the dogs, and one dog ate the case it was in.