*An idea is not a plot*. It begins with a "one-liner" - a single sentence or a visual image.
The most obvious source of inspiration is your own hobby or profession or job specialty. Hobbies, cats, dogs, reading, computer, provide great inspiration for ideas. They can provide clues, plot twists, many things. Sometimes irritation spawns a plot idea. Sometimes, when a smashing twist in another book I'm reading doesn't quite come off, I say, "I can do that better." And you do.
When looking for a plot idea, is is helpful to read as widely as possible. Visual images can also be helpful: a handsome old house, a quaint village, a medieval town. Misinterpreted images can also be helpful: like when you wake up in the middle of the night and see some familiar object in the room transformed by shadows and moonlight. Remarks heard on planes or buses, unusual signs in shop windows, street names, etc. Ideas are everywhere. Satires and takeoffs, like being at a writer's conference and hearing something funny or completely off the wall. Even church meetings could inspire something funny.
Write your idea's down on notecards and file them away for later.
*Another file: "miscellaneious" could contain newpaper clippings, supernatural stuff, crime.*
The idea is to encourage the initial image or brief sentence to develop into something a little more substantial.
*Remember, an idea is not a plot.*
Another way to come up with ideas is to find an ambiance in which your mind is free to wander. Mild physical activity, such as walking, some types of housework, such as ironing, necessitate a blank mind - ideal for the mind to wander freely.