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Twilight sky would be a good source. Computer screen less so (have a look at the colour cast at the top of the frame compared to the bottom, not to mention backlights on displays are not uniform).
You need to match the flat to the lens and the orientation. It's easy for me since I mount the camera the same way every time and I've been using the same set of flats for multiple shoots. What I did is took a photo of the wall of my house through the telescope at a time and place where there was no light gradient (bathroom was the best since it's all white).
But if you prep your frames early like this rather than shooting right after or during your lightframes then they will do nothing for dust removal. Worse still if there's dust in your flats which isn't in your lights then you'll introduce nasty looking bright blotches in your images.
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